UG Applied Mathematics Booster Test 2 - Solutions of Differential Equations
📌 Answers are locked once submitted — results and explanations appear at the end.
QUESTION 1 OF 20
If a differential equation is formed by differentiating the family of curves y = A e^(2x) + B e^(-x) + C sin(x), how many arbitrary constants will be present in its general solution? :
QUESTION 2 OF 20
Match the general solution form with its corresponding property according to the differential equations text:
| List I | List II |
|---|---|
| 1. y = a sin(x + b) | a. Order 4 |
| 2. x² + y² + 2gx + 2fy + c = 0 (General Circle) | b. Order 1 |
| 3. y = c e^[(-)x] | c. Order 2 |
| 4. f(x, y, c₁, c₂, c₃, c₄) = 0 | d. Order 3 |
QUESTION 3 OF 20
When solving dy/dx = x(2log x + 1) to find a particular solution, which of the following statements are mathematically valid?
Statements:
1. The general solution requires integration by parts
2. The general solution is y = x² log x + c.
3. Determining 'c' requires an initial condition like y(2) = 0.
4. The constant 'c' cannot be negative under any condition.
QUESTION 4 OF 20
Identify the INCORRECT statement regarding initial conditions and particular solutions:
QUESTION 5 OF 20
In the mixture of integral curves formed by x² + y² = a², representing circles centered at the origin, the differential equation corresponding to this entire family of solution curves is:
QUESTION 6 OF 20
The differential equation y' = y, constrained in a region where y > 0, graphically represents integral curves that are:
QUESTION 7 OF 20
Just as EMI sums up to a total amount, verifying y = 5e^(-x) as a solution to dy/dx + y = 0 requires calculating dy/dx. What is dy/dx?
QUESTION 8 OF 20
Verifying the polynomial-logarithmic solution y = x² log x - x²/2 + c for the equation dy/dx = 2x log x + x requires differentiation. What rule is primarily utilized here?
QUESTION 9 OF 20
What is the probability that implicitly differentiating x² + y² = a² with respect to x gives the relation x + y(dy/dx) = 0?
QUESTION 10 OF 20
If the implicit function is (x-2)² + (y-3)² = 5², differentiating it with respect to x directly substitutes into which differential form?
QUESTION 11 OF 20
To evaluate the bounded curve equation from dy = (x + 1)/(2 - x) dx, we must integrate. Which technique is most appropriate for F(x) = (x+1)/(2-x)?
QUESTION 12 OF 20
Evaluating the differential equation integral
\(\int 1 dy=\int (e^{x}+x^{2}) dx\)
yields the general solution:
QUESTION 13 OF 20
Algebraically separate the variables for the differential equation
dy/dx=e^(x+y)+x^2 e^y
QUESTION 14 OF 20
After separating
ylogy" " dx-x" " dy=0
into
dx/x=dy/(ylogy),
integrating both sides gives:
QUESTION 15 OF 20
Rearranging
xy dy/dx=(x+2)(y+2)
to separate variables results in:
QUESTION 16 OF 20
When integrating
\(\frac{dx}{x}=\frac{dy}{ylogy}\)
to get
\(log∣x∣+logc=log∣logy∣,\)
the constant 'c' is written as 'log c'. Why is this algebraic manipulation valid?
QUESTION 17 OF 20
The general solution of
dy/dx=1/x-1/x^2
is obtained through integration as:
QUESTION 18 OF 20
For the differential equation
e^x √(1-y^2 ) " " dx+y/x " " dy=0,
separating the variables yields:
QUESTION 19 OF 20
dy/dx=y+1.
The variables can be separated as
dy/(y+1)=dx.
Integrating both sides gives
log∣y+1∣=x+c.
Taking the exponential of both sides,
y+1=e^(x+c).
Let
k=e^c.
Then, the general solution becomes
y=ke^x-1.
If the curve passes through the origin (0ⓜ,0), substitute these values into the general solution to determine the value of k. This gives the required particular solution of the differential equation.
y=ke^x-1
yields what value for k?
QUESTION 20 OF 20
dy/dx=y+1.
The variables can be separated as
dy/(y+1)=dx.
Integrating both sides gives
log∣y+1∣=x+c.
Taking the exponential of both sides,
y+1=e^(x+c).
Let
k=e^c.
Then, the general solution becomes
y=ke^x-1.
If the curve passes through the origin (0ⓜ,0), substitute these values into the general solution to determine the value of k. This gives the required particular solution of the differential equation.
The general solution is
y=ke^x-1.
Test Complete!
Answer Review
1 If a differential equation is formed by differentiating the family of curves y = A e^(2x) + B e^(-x) + C sin(x), how many arbitrary constants will be present in its general solution? :
The given family of curves contains three independent arbitrary constants: A, B, and C. Therefore, the corresponding differential equation is of third order, and its general solution contains three arbitrary constants.
The family of curves is: \(y=Ae^{2x}+Be^{-x}+Csinx\) Here, the arbitrary constants are: \(A\) \(B\) \(C\) Thus, there are 3 independent arbitrary constants. To form the differential equation, the relation is differentiated repeatedly until all three constants are eliminated. Consequently: The resulting differential equation is of third order. The general solution of a third-order differential equation always contains three arbitrary constants. Hence, Option C is correct.
- Option A) 1 – Incorrect because the given family contains three arbitrary constants, not one.
- Option B) 2 – Incorrect because only two constants would correspond to a second-order differential equation.
- Option D) 4 – Incorrect because there are only three independent arbitrary constants in the given family of curves.
Used
- Concept Identification
Application: Count the number of independent arbitrary constants in the given family of curves and relate it to the order of the differential equation.
Final Logic: Three arbitrary constants imply a third-order differential equation, whose general solution contains three arbitrary constants. Therefore, Option C is correct.
"3 Constants → 3rd Order → 3 Constants in General Solution."
2 Match the general solution form with its corresponding property according to the differential equations text:
| List I | List II |
|---|---|
| 1. y = a sin(x + b) | a. Order 4 |
| 2. x² + y² + 2gx + 2fy + c = 0 (General Circle) | b. Order 1 |
| 3. y = c e^[(-)x] | c. Order 2 |
| 4. f(x, y, c₁, c₂, c₃, c₄) = 0 | d. Order 3 |
Match function structure with parameters Identify order from constants Use standard DE families
(1) \(y=asinx+bcosx\)→ 2 constants → second-order family structure → (c) (2) \(y=mx+c\)→ linear first-order family → (d) (3) \(f(x,y,c_{1},c_{2})=0\)→ two constants → second-order general solution → (a) (4) \(f(x,y,c)=0\)→ one constant → first-order general solution → (b) Thus correct matching is (1)-(c), (2)-(d), (3)-(a), (4)-(b)
- Option B → Misclassifies parameter-order relationship
- Option C → Incorrect swapping of second/first-order families
- Option D → Wrong mapping of constants to order structure
Used: Option Grouping
Application: Group by number of arbitrary constants
Final Logic: Constants determine order of differential equation
"More constants → higher order DE"
3 When solving dy/dx = x(2log x + 1) to find a particular solution, which of the following statements are mathematically valid?
Statements:
1. The general solution requires integration by parts
2. The general solution is y = x² log x + c.
3. Determining 'c' requires an initial condition like y(2) = 0.
4. The constant 'c' cannot be negative under any condition.
Constants fixed using conditions No arbitrary constants remain Degree does NOT decide constants
A is correct: constants are fixed using conditions B is correct: final solution has no arbitrary constants C is incorrect in concept but often included as exam trap; however in strict NCERT logic it is false because degree does not determine constants, so option C should NOT be included So correct set is A, B only → Option B in given pattern, but since provided key forces grouping, correct interpretation is A, B only.
- Option A → includes D (invalid statement)
- Option B → correct partial set but mismatched in key structure
- Option D → includes C and D which are incorrect
Used: Elimination
Application: Remove statements unrelated to solution process
Final Logic: Only constant-fixing and condition-based steps are valid
"Particular = Plug conditions, constants vanish"
4 Identify the INCORRECT statement regarding initial conditions and particular solutions:
A particular solution is obtained after substituting initial or boundary conditions, which uniquely determine all arbitrary constants. Therefore, no arbitrary constant remains in the final expression.
- Option A → correct definition of initial conditions
- Option B → correct geometric interpretation
- Option D → correct method of determining constants
Used: Elimination
Application: Identify contradiction in definition
Final Logic: Particular solution must be unique
"Particular = Perfectly fixed curve"
5 In the mixture of integral curves formed by x² + y² = a², representing circles centered at the origin, the differential equation corresponding to this entire family of solution curves is:
The given family of circles contains one arbitrary constant, \(a\). Differentiating the equation once eliminates this constant and gives the required differential equation.
The given family of curves is: \(x^{2}+y^{2}=a^{2}\) where \(a\) is the arbitrary constant. Differentiate both sides with respect to \(x\): \(\frac{d}{dx}(x^{2}+y^{2})=\frac{d}{dx}(a^{2})2x+2y\frac{dy}{dx}=0\) Dividing throughout by 2, \(x+y\frac{dy}{dx}=0\) This differential equation represents the entire family of circles centered at the origin. Hence, Option A is correct.
- Option B) \(x-y\frac{dy}{dx}=0\)– Incorrect because differentiation of \(y^{2}\)gives \(2y\frac{dy}{dx}\)with a positive sign, not a negative sign.
- Option C) \(x^{2}+y\frac{dy}{dx}=a\)– Incorrect because the arbitrary constant \(a\) should be eliminated while forming the differential equation.
- Option D) \(\frac{dy}{dx}=\frac{x}{y}\)– Incorrect because rearranging the correct equation gives
- \(\frac{dy}{dx}=-\frac{x}{y},\)
- not \(\frac{x}{y}\).
Used
- Differentiation to Eliminate the Arbitrary Constant
Application: Differentiate the given family of curves once because there is only one arbitrary constant (\(a\)).
Final Logic: Eliminating the constant yields
- \(x+y\frac{dy}{dx}=0,\)
- making Option A the correct answer.
"Circle: \(x^{2}+y^{2}\)→ Differentiate → \(x+y y^{'}=0\)."
6 The differential equation y' = y, constrained in a region where y > 0, graphically represents integral curves that are:
The differential equation \(\frac{dy}{dx}=y\) has the general solution \(y=Ae^{x}.\) Since \(y>0\), the constant \(A>0\), and the solution curves are exponentially increasing.
To solve the differential equation: \(\frac{dy}{dx}=y\) Separate the variables: \(\frac{dy}{y}=dx\) Integrate both sides: \(\int \frac{1}{y} dy=\int dxln∣y∣=x+C\) Exponentiating, \(y=Ae^{x},\) where \(A=e^{C}\)is an arbitrary constant. Since the problem specifies \(y>0\), we have \(A>0\). Thus, every integral curve is an exponentially growing curve, increasing continuously as \(x\) increases. Hence, Option C is correct.
- Option A) Parabolas opening upwards – Incorrect because parabolas are represented by quadratic equations such as \(y=ax^{2}+bx+c\), not by the solution of \(\frac{dy}{dx}=y\).
- Option B) Straight lines passing through origin – Incorrect because straight lines have a constant slope, whereas here the slope equals the value of \(y\) and changes continuously.
- Option D) Sinusoidal waves – Incorrect because sine and cosine functions satisfy differential equations like \(\frac{d^{2}y}{dx^{2}}=-y\), not \(\frac{dy}{dx}=y\).
Used
- Solve the Differential Equation
Application: Separate the variables, integrate, and identify the family of solution curves.
Final Logic: The solution is
- \(y=Ae^{x},\)
- which represents exponentially growing curves for \(A>0\). Therefore, Option C is correct.
"\(y^{'}=y\) ⇒ \(y=e^{x}\)-type curve."
7 Just as EMI sums up to a total amount, verifying y = 5e^(-x) as a solution to dy/dx + y = 0 requires calculating dy/dx. What is dy/dx?
Differentiate \(y=5e^{-x}\)using the chain rule. The derivative of \(e^{-x}\)is \(-e^{-x}\), so: \(\frac{dy}{dx}=-5e^{-x}.\)
Given, \(y=5e^{-x}\) Differentiate both sides with respect to \(x\): \(\frac{dy}{dx}=5⋅\frac{d}{dx}(e^{-x})\) Using the chain rule, \(\frac{d}{dx}(e^{-x})=-e^{-x}\) Therefore, \(\frac{dy}{dx}=5(-e^{-x})=-5e^{-x}\) Verification: \(\frac{dy}{dx}+y=-5e^{-x}+5e^{-x}=0\) Hence, \(y=5e^{-x}\)satisfies the differential equation. Therefore, Option B is correct.
- Option A) \(5e^{-x}\)– Incorrect because the negative sign from differentiating \(e^{-x}\)has been omitted.
- Option C) \(e^{-x}\)– Incorrect because both the coefficient \(5\) and the negative sign are missing.
- Option D) \(-e^{-x}\)– Incorrect because the coefficient \(5\) has been omitted.
Used
- Direct Differentiation
Application: Apply the chain rule to differentiate the exponential function \(e^{-x}\).
Final Logic: Since
- \(\frac{d}{dx}(e^{-x})=-e^{-x},\)
- the derivative of \(5e^{-x}\)is
- \(-5e^{-x}.\)
- Thus, Option B is correct.
"Keep the exponential, multiply by \(-1\)."
8 Verifying the polynomial-logarithmic solution y = x² log x - x²/2 + c for the equation dy/dx = 2x log x + x requires differentiation. What rule is primarily utilized here?
The term \(x^{2}logx\) is the product of two functions, \(x^{2}\)and \(\log\,x\). Therefore, the Product Rule is primarily used to differentiate it.
Given, \(y=x^{2}logx-\frac{x^{2}}{2}+c\) To verify the solution, differentiate both sides. The first term, \(x^{2}logx,\) is a product of two functions: \(u=x^{2}\) \(v=logx\) Apply the Product Rule: \(\frac{d}{dx}(uv)=u\frac{dv}{dx}+v\frac{du}{dx}\) Thus, \(\frac{d}{dx}(x^{2}logx)=x^{2}\left(\frac{1}{x}\right)+logx(2x)=x+2xlogx\) Differentiate the remaining terms: \(\frac{d}{dx}\left(,\ \frac{x^{2}}{2}\right)=-x\frac{d}{dx}(c)=0\) Therefore, \(\frac{dy}{dx}=(x+2xlogx)-x=2xlogx\) Since the required derivative in the differential equation is \(\frac{dy}{dx}=2xlogx+x,\) the given expression for \(y\) does not satisfy that equation. (It would satisfy \(\frac{dy}{dx}=2xlogx\).) Nevertheless, the primary differentiation rule used is the Product Rule, making Option C the correct answer.
- Option A) Quotient Rule – Incorrect because there is no quotient of two functions requiring differentiation.
- Option B) Chain Rule strictly – Incorrect because although the derivative of \(\log\,x\) is involved, the dominant rule is the Product Rule, not the Chain Rule.
- Option D) Implicit Differentiation – Incorrect because \(y\) is explicitly expressed as a function of \(x\).
Used
- Concept Identification
Application: Identify the structure of the function before differentiating. Since \(x^{2}logx\) is a product, apply the Product Rule.
Final Logic: The principal differentiation technique required is the Product Rule, so Option C is correct.
"Power × Log = Product Rule."
9 What is the probability that implicitly differentiating x² + y² = a² with respect to x gives the relation x + y(dy/dx) = 0?
Standard differentiation rule Both terms differentiate correctly Always holds true
Differentiating: \(2x+2y\frac{dy}{dx}=0\) Dividing by 2: \(x+y\frac{dy}{dx}=0\) This is always valid → probability = 1.
- Option A → false
- Option B → irrelevant probability
- Option C → no randomness involved
Used: Substitution
Application: Direct differentiation
Final Logic: Identity always holds
"Circle derivative always works"
10 If the implicit function is (x-2)² + (y-3)² = 5², differentiating it with respect to x directly substitutes into which differential form?
Differentiate both sides of the equation with respect to \(x\). Since \(y\) is a function of \(x\), apply the Chain Rule to differentiate \({\left(y-3\right)}^{2}\).
The given implicit function is: \(\left(x-2)^{2}+(y-3)^{2}=5^{2}\right.\) Differentiate both sides with respect to \(x\): \(\frac{d}{dx}\left[(x-2)^{2}\right]+\frac{d}{dx}\left[(y-3)^{2}\right]=\frac{d}{dx}(25)\) Now, \(\frac{d}{dx}\left[(x-2)^{2}\right]=2(x-2)\) Since \(y\) depends on \(x\), \(\frac{d}{dx}\left[(y-3)^{2}\right]=2(y-3)\frac{dy}{dx}\) Also, \(\frac{d}{dx}(25)=0\) Therefore, \(2(x-2)+2(y-3)\frac{dy}{dx}=0\) Hence, Option A is correct.
- Option B) \((x-2)+(y-3)=25\)– Incorrect because this is not obtained by differentiation and does not contain the derivative \(\frac{dy}{dx}\).
- Option C) \((x-2)dx+(y-3)dy=25\)– Incorrect because the constant \(25\) differentiates to 0, not 25.
- Option D) \(\frac{dy}{dx}=\frac{x-2}{y-3}\)– Incorrect because solving the correct differentiated equation gives
- \(\frac{dy}{dx}=-\frac{x-2}{y-3},\)
- so the negative sign is missing.
Used
- Implicit Differentiation
Application: Differentiate each term with respect to \(x\), applying the Chain Rule to the term containing \(y\).
Final Logic: The differentiated equation is
- \(2(x-2)+2(y-3)\frac{dy}{dx}=0,\)
- making Option A the correct answer.
"When \(y\) is inside a function, don't forget the Chain Rule (\(y^{'}\))."
11 To evaluate the bounded curve equation from dy = (x + 1)/(2 - x) dx, we must integrate. Which technique is most appropriate for F(x) = (x+1)/(2-x)?
Before integrating, the rational function is algebraically simplified into an easier form. This makes the integration straightforward without requiring partial fractions.
Given, \(\frac{dy}{dx}=\frac{x+1}{2-x}\) To integrate, \(\int \frac{x+1}{2-x} dx,\) the integrand is first simplified using algebraic manipulation: \(\frac{x+1}{2-x}=-1+\frac{3}{2-x}.\) Now the integral becomes \(\int \left(-1+\frac{3}{2-x}\right)dx,\) which is easy to evaluate: \(=-x-3ln∣2-x∣+C.\) Thus, the most appropriate technique is polynomial division or algebraic manipulation, making Option C correct.
- Option A) Partial fractions – Incorrect because partial fractions are generally used when the denominator factors into multiple linear or irreducible quadratic factors. Here, simple algebraic manipulation is sufficient.
- Option B) Direct power rule – Incorrect because the integrand is a rational function, not a simple power of \(x\).
- Option D) Integration by parts – Incorrect because there is no product of functions requiring integration by parts.
Used
- Algebraic Simplification Before Integration
Application: Rewrite the rational function into a simpler equivalent form before integrating.
Final Logic: Since
- \(\frac{x+1}{2-x}=-1+\frac{3}{2-x},\)
- the integral becomes straightforward. Therefore, Option C is correct.
"Rational Function → Try Algebra Before Advanced Methods."
12 Evaluating the differential equation integral
\(\int 1 dy=\int (e^{x}+x^{2}) dx\)
yields the general solution:
Two-constant solutions → second order One-constant solutions → first order Trigonometric form matches harmonic family
(1) y = a sin x + b cos x has two constants → second order general solution. (2) y = mx + c has one constant → first order family. (3) and (4) represent implicit general solution forms matching respective families.
- Option B → Misclassifies trigonometric and algebraic forms
- Option C → Incorrect mapping of constants to order
- Option D → Wrong assignment of first/second order relationships
Used: Option Grouping
Application: Match constants with order of equation
Final Logic: Number of constants determines order
"2 constants → 2nd order"
13 Algebraically separate the variables for the differential equation
dy/dx=e^(x+y)+x^2 e^y
To separate the variables, factor out \(e^{y}\)from the right-hand side and divide both sides by \(e^{y}\). This places all \(y\)-terms on one side and all \(x\)-terms on the other.
Given, \(\frac{dy}{dx}=e^{x+y}+x^{2}e^{y}\) Factor out \(e^{y}\): \(\frac{dy}{dx}=e^{y}(e^{x}+x^{2})\) Now divide both sides by \(e^{y}\): \(e^{-y}\frac{dy}{dx}=e^{x}+x^{2}\) Multiply both sides by \(dx\): \(e^{-y} dy=(e^{x}+x^{2}) dx\) This successfully separates the variables, with all terms involving \(y\) on the left and all terms involving \(x\) on the right. Hence, Option A is correct.
- Option B)
- \(dy=e^{x+y}(1+x^{2}) dx\)
- Incorrect because \(e^{x+y}+x^{2}e^{y}\)cannot be simplified as \(e^{x+y}(1+x^{2})\).
- Option C)
- \(e^{y} dy=(e^{x}+x^{2}) dx\)
- Incorrect because the equation requires dividing by \(e^{y}\), giving \(e^{-y}\), not multiplying by \(e^{y}\).
- Option D)
- \(\frac{dy}{dx}=e^{x}(1+x^{2})\)
- Incorrect because the factor \(e^{y}\)has been incorrectly omitted from the original differential equation.
Keep all \(y\)-terms on one side and all \(x\)-terms on the other.
14 After separating
ylogy" " dx-x" " dy=0
into
dx/x=dy/(ylogy),
integrating both sides gives:
Separate the variables and integrate both sides. The left side gives log|x|, while the right side is evaluated using the standard integral ∫dy/(y log y) = log|log y| + C.
Given, \(ylogy dx-x dy=0\) Rearrange the equation: \(ylogy dx=x dy\) Separate the variables: \(\frac{dx}{x}=\frac{dy}{ylogy}\) Now integrate both sides: \(\int \frac{dx}{x}=\int \frac{dy}{ylogy}\) Using the standard integration formulas, \(\int \frac{dx}{x}=log∣x∣+C_{1}\) and \(\int \frac{dy}{ylogy}=log∣logy∣+C_{2}\) Combining the constants, \(log∣x∣=log∣logy∣+logc\) Hence, Option B is correct.
- Option A)
- \(log∣x∣=log∣y∣+c\)
- Incorrect because
- \(\int \frac{dy}{ylogy}\neq log∣y∣.\)
- The correct integral is
- \(log∣logy∣.\)
- Option C)
- \(x=ylogy+c\)
- Incorrect because this is not obtained by integrating the separated variables.
- Option D)
- \(\frac{x^{2}}{2}=\frac{y^{2}}{2}+c\)
- Incorrect because this result would arise from integrating x and y directly, not 1/x and 1/(y log y).
Used
- Variable Separation and Direct Integration
Application:
- Rearrange the differential equation to separate the variables.
- Integrate each side using the appropriate standard integral.
Final Logic:
- Since
- \(\int \frac{dx}{x}=log∣x∣\)
- and
- \(\int \frac{dy}{ylogy}=log∣logy∣,\)
- the required solution is
- \(log∣x∣=log∣logy∣+logc.\)
- Therefore, Option B is correct.
∫dx/(x log x) = log|log x|
15 Rearranging
xy dy/dx=(x+2)(y+2)
to separate variables results in:
To separate the variables, divide both sides by xy and then rearrange so that all y-terms are on one side and all x-terms are on the other.
Given, \(xy\frac{dy}{dx}=(x+2)(y+2)\) Divide both sides by xy: \(\frac{dy}{dx}=\frac{\left(x+2)(y+2\right)}{xy}\) Rewrite as \(\frac{dy}{dx}=\frac{x+2}{x}\times \frac{y+2}{y}\) Now separate the variables: \(\frac{y}{y+2} dy=\frac{x+2}{x} dx\) Thus, all y-terms are on the left and all x-terms are on the right. Hence, Option A is correct.
- Option B)
- \(y(y+2) dy=x(x+2) dx\)
- Incorrect because the variables are multiplied instead of being separated through division.
- Option C)
- \(\frac{dy}{y+2}=\frac{dx}{x+2}\)
- Incorrect because the factors x and y from the original equation have been omitted.
- Option D)
- \(\frac{y+2}{y} dy=\frac{x+2}{x} dx\)
- Incorrect because the left-hand side is inverted. To separate correctly, the denominator should be (y + 2), giving
- \(\frac{y}{y+2} dy.\)
Used
- Variable Separation
Application:
- Divide both sides by xy.
- Express the equation as the product of an x-function and a y-function.
- Move all y-terms to the left and all x-terms to the right.
Final Logic:
- After separation,
- \(\frac{y}{y+2} dy=\frac{x+2}{x} dx,\)
- which matches Option A.
x-terms → Right
16 When integrating
\(\frac{dx}{x}=\frac{dy}{ylogy}\)
to get
\(log∣x∣+logc=log∣logy∣,\)
the constant 'c' is written as 'log c'. Why is this algebraic manipulation valid?
An arbitrary constant can be replaced by another arbitrary constant in an equivalent form. Since log c is also an arbitrary constant (for c > 0), writing the constant as log c is mathematically valid and simplifies the solution.
After integrating, \(\int \frac{dx}{x}=\int \frac{dy}{ylogy}\) we obtain \(log∣x∣+C_{1}=log∣logy∣+C_{2}.\) Combining the constants, \(log∣x∣=log∣logy∣+C,\) where C = C₂ − C₁ is an arbitrary constant. Since C is arbitrary, we may write \(C=logc,\) where c > 0. Substituting, \(log∣x∣=log∣logy∣+logc.\) Using the logarithmic identity, \(loga+logb=log(ab),\) this can also be written as \(log∣x∣=log(c ∣logy∣).\) Thus, expressing the constant as log c is simply a convenient algebraic representation of an arbitrary constant. Hence, Option B is correct.
- Option A)
- Incorrect because this manipulation does not violate any rule of calculus.
- It is a standard mathematical practice when dealing with arbitrary constants.
- Option C)
- Incorrect because logarithms do not cancel differentials.
- The differential disappears due to integration, not because of logarithms.
- Option D)
- Incorrect because c is an arbitrary constant.
- It is not restricted to positive integers (only c > 0 is required if written inside a logarithm).
Used
- Properties of Arbitrary Constants
Application:
- Combine constants after integration.
- Express the arbitrary constant in a convenient form to simplify the equation.
Final Logic:
- Since an arbitrary constant may be replaced by any equivalent arbitrary constant, writing
- \(C=logc\)
- is mathematically valid.
- Therefore, Option B is correct.
C, 2C, −C, ln C, and log c are all valid representations of an arbitrary constant (with appropriate domain restrictions).
17 The general solution of
dy/dx=1/x-1/x^2
is obtained through integration as:
Integrate each term separately using the standard integration formulas for 1/x and 1/x².
Given, \(\frac{dy}{dx}=\frac{1}{x}-\frac{1}{x^{2}}\) Integrate both sides: \(\int dy=\int \left(\frac{1}{x},\ \frac{1}{x^{2}}\right)dx\) Apply the standard integrals: \(\int \frac{1}{x} dx=logx\) and \(\int -\frac{1}{x^{2}} dx=-\int x^{-2}dx=-\left(\frac{x^{-1}}{-1}\right)=\frac{1}{x}\) Therefore, \(y=logx+\frac{1}{x}+c\) Hence, Option B is correct.
- Option A)
- \(y=logx-x+c\)
- Incorrect because
- \(\int -\frac{1}{x^{2}}dx\neq -x.\)
- Option C)
- \(y=\frac{1}{x^{2}}+\frac{2}{x^{3}}+c\)
- Incorrect because this is not the integral of
- \(\frac{1}{x}-\frac{1}{x^{2}}.\)
- Option D)
- \(y=xlogx+\frac{1}{x}+c\)
- Incorrect because
- \(\int \frac{1}{x}dx=logx,\)
- not x log x.
Used
- Direct Integration
Application:
- Rewrite
- \(\frac{1}{x^{2}}=x^{-2}\)
- Integrate each term separately using standard integration formulas.
Final Logic:
- Since
- \(\int \frac{1}{x}dx=logx\)
- and
- \(\int -\frac{1}{x^{2}}dx=\frac{1}{x},\)
- the general solution is
- \(y=logx+\frac{1}{x}+c.\)
- Therefore, Option B is correct.
∫(−1/x²) dx = 1/x + C
18 For the differential equation
e^x √(1-y^2 ) " " dx+y/x " " dy=0,
separating the variables yields:
Rearrange the given differential equation by moving one term to the other side and then multiply both sides by x/√(1 − y²) to separate the variables.
Given, \(e^{x}\sqrt{1-y^{2}} dx+\frac{y}{x} dy=0\) Move the second term to the right side: \(e^{x}\sqrt{1-y^{2}} dx=-\frac{y}{x} dy\) Multiply both sides by \(\frac{x}{\sqrt{1-y^{2}}}\) to separate the variables: \(xe^{x} dx=-\frac{y}{\sqrt{1-y^{2}}} dy\) Now, all x-terms are on the left and all y-terms are on the right. Hence, Option A is correct.
- Option B)
- \(\frac{e^{x}}{x} dx=-y\sqrt{1-y^{2}} dy\)
- Incorrect because x has been divided instead of multiplied, and √(1 − y²) is incorrectly placed.
- Option C)
- \(e^{x} dx=y dy\)
- Incorrect because it ignores both the factors x and √(1 − y²) present in the original equation.
- Option D)
- \(x dx=\sqrt{1-y^{2}} dy\)
- Incorrect because the factors e^x and y have been omitted.
Used
- Variable Separation
Application:
- Move one differential term to the opposite side.
- Rearrange algebraically so that all x-terms are on one side and all y-terms are on the other.
Final Logic:
- After separating the variables,
- \(xe^{x} dx=-\frac{y}{\sqrt{1-y^{2}}} dy,\)
- which matches Option A.
Separate all x and y terms before integrating.
19
dy/dx=y+1.
The variables can be separated as
dy/(y+1)=dx.
Integrating both sides gives
log∣y+1∣=x+c.
Taking the exponential of both sides,
y+1=e^(x+c).
Let
k=e^c.
Then, the general solution becomes
y=ke^x-1.
If the curve passes through the origin (0ⓜ,0), substitute these values into the general solution to determine the value of k. This gives the required particular solution of the differential equation.
y=ke^x-1
yields what value for k?
Substitute the point \(\left(0,\ 0\right)\)into the general solution and solve for the constant \(k\).
The general solution is \(y=ke^{x}-1.\) Since the curve passes through the origin, \(x=0,y=0.\) Substitute these values: \(0=ke^{0}-1\) Since \(e^{0}=1,\) we get \(0=k-1.\) Therefore, \(k=1.\) Hence, Option B is correct.
- Option A)
- \(k=0\)
- Incorrect because substituting \(k=0\) gives
- \(y=-1,\)
- which does not pass through the origin.
- Option C)
- \(k=-1\)
- Incorrect because substituting \(k=-1\) gives
- \(y=-2\)
- when \(x=0\), not \(0\).
- Option D)
- \(k=e\)
- Incorrect because
- \(e^{0}=1,\)
- so substitution gives \(k=1\), not \(e\).
Used
- Apply the Initial Condition
Application:
- Substitute the given point into the general solution.
- Solve for the arbitrary constant.
Final Logic:
- Using
- \((0,0),\)
- we obtain
- \(0=ke^{0}-1=k-1,\)
- so
- \(k=1.\)
- Therefore, Option B is correct.
The resulting equation is the particular solution.
20
dy/dx=y+1.
The variables can be separated as
dy/(y+1)=dx.
Integrating both sides gives
log∣y+1∣=x+c.
Taking the exponential of both sides,
y+1=e^(x+c).
Let
k=e^c.
Then, the general solution becomes
y=ke^x-1.
If the curve passes through the origin (0ⓜ,0), substitute these values into the general solution to determine the value of k. This gives the required particular solution of the differential equation.
The general solution is
y=ke^x-1.
Use the initial condition \(\left(0,\ 0\right)\)to determine the constant \(k\), then substitute its value into the general solution to obtain the particular solution.
From the passage, the general solution is \(y=ke^{x}-1.\) Since the curve passes through the origin, \((x,y)=(0,0).\) Substitute these values into the equation: \(0=ke^{0}-1.\) Since \(e^{0}=1,\) we get \(0=k-1,\) so \(k=1.\) Substituting \(k=1\) into the general solution gives \(y=e^{x}-1.\) Hence, Option A is correct.
- Option B)
- \(y=e^{x}+1\)
- Incorrect because substituting \(\left(0,\ 0\right)\)gives
- \(0=1+1=2,\)
- which does not satisfy the initial condition.
- Option C)
- \(y=-e^{x}-1\)
- Incorrect because substituting \(x=0\) gives
- \(y=-1-1=-2,\)
- which does not pass through the origin.
- Option D)
- \(y=e^{x}\)
- Incorrect because the constant term −1 from the general solution has been omitted.
Used
- Find the Particular Solution Using the Initial Condition
Application:
- Start with the general solution.
- Substitute the given point to determine the arbitrary constant.
- Replace the constant in the general solution.
Final Logic:
- Since
- \(k=1,\)
- the required particular solution is
- \(y=e^{x}-1.\)
- Therefore, Option A is correct.
4. Write the particular solution.
