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Differential Equations question

2016 · Shift 1 · Q32
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  5. /2016 · Shift 1 · Q32

Differential Equations question

2016 · Shift 1 · Q32

JEE AdvancedMathematicsDifferential EquationsMultiple correct+4 / −2
Let f:(0,∞)→Rf:(0,\infty ) \to Rf:(0,∞)→R be a differentiable function such that f′(x)=2−f(x)xf'(x) = 2 - {{f(x)} \over x}f′(x)=2−xf(x)​ for all x∈(0,∞)x \in (0,\infty )x∈(0,∞) and f(1)e1f(1) e 1f(1)e1. Then
  1. A
    lim⁡x→0+f′(1x)=1\mathop {\lim }\limits_{x \to {0^ + }} f'\left( {{1 \over x}} \right) = 1x→0+lim​f′(x1​)=1
  2. B
    lim⁡x→0+xf(1x)=2\mathop {\lim }\limits_{x \to {0^ + }} xf\left( {{1 \over x}} \right) = 2x→0+lim​xf(x1​)=2
  3. C
    lim⁡x→0+x2f′(x)=0\mathop {\lim }\limits_{x \to {0^ + }} {x^2}f'(x) = 0x→0+lim​x2f′(x)=0
  4. D
    ∣f(x)∣≤2\left| {f(x)} \right| \le 2∣f(x)∣≤2 for all x∈(0,2)x \in (0,2)x∈(0,2)
View written solutionFree

Correct answer: A

Step 1: Solve the Differential Equation

The given differential equation is f′(x)=2−f(x)xf'(x) = 2 - \frac{f(x)}{x}f′(x)=2−xf(x)​. We can rewrite this as a first-order linear differential equation: f′(x)+1xf(x)=2f'(x) + \frac{1}{x}f(x) = 2f′(x)+x1​f(x)=2 This is in the form y′+P(x)y=Q(x)y' + P(x)y = Q(x)y′+P(x)y=Q(x), with y=f(x)y = f(x)y=f(x), P(x)=1xP(x) = \frac{1}{x}P(x)=x1​, and Q(x)=2Q(x) = 2Q(x)=2.

The integrating factor (I.F.) is calculated as: I.F.=e∫P(x)dx=e∫1xdx=eln⁡x=xI.F. = e^{\int P(x) dx} = e^{\int \frac{1}{x} dx} = e^{\ln x} = xI.F.=e∫P(x)dx=e∫x1​dx=elnx=x (since x∈(0,∞)x \in (0, \infty)x∈(0,∞)).

Multiply the differential equation by the integrating factor: xf′(x)+x⋅1xf(x)=2xx f'(x) + x \cdot \frac{1}{x} f(x) = 2xxf′(x)+x⋅x1​f(x)=2x xf′(x)+f(x)=2xx f'(x) + f(x) = 2xxf′(x)+f(x)=2x The left side is the derivative of the product of f(x)f(x)f(x) and the integrating factor xxx: ddx(xf(x))=2x\frac{d}{dx}(x f(x)) = 2xdxd​(xf(x))=2x

Integrate both sides with respect to xxx: ∫ddx(xf(x))dx=∫2xdx\int \frac{d}{dx}(x f(x)) dx = \int 2x dx∫dxd​(xf(x))dx=∫2xdx xf(x)=x2+Cx f(x) = x^2 + Cxf(x)=x2+C where CCC is the constant of integration.

The general solution for f(x)f(x)f(x) is: f(x)=x+Cxf(x) = x + \frac{C}{x}f(x)=x+xC​

To verify, let's find the derivative of our solution: f′(x)=1−Cx2f'(x) = 1 - \frac{C}{x^2}f′(x)=1−x2C​ Substitute f(x)f(x)f(x) and f′(x)f'(x)f′(x) into the original equation: 1−Cx2=2−x+C/xx=2−(1+Cx2)=1−Cx21 - \frac{C}{x^2} = 2 - \frac{x + C/x}{x} = 2 - (1 + \frac{C}{x^2}) = 1 - \frac{C}{x^2}1−x2C​=2−xx+C/x​=2−(1+x2C​)=1−x2C​. The solution is correct.

Step 2: Interpret the Initial Condition

The condition given is f(1) e 1f(1) \text{ e } 1f(1) e 1. This is likely a typographical error for f(1)≠1f(1) \neq 1f(1)=1. Let's proceed with this assumption. Using our solution, we find f(1)f(1)f(1): f(1)=1+C1=1+Cf(1) = 1 + \frac{C}{1} = 1 + Cf(1)=1+1C​=1+C The condition f(1)≠1f(1) \neq 1f(1)=1 implies 1+C≠11 + C \neq 11+C=1, which means C≠0C \neq 0C=0.

So we have the function f(x)=x+Cxf(x) = x + \frac{C}{x}f(x)=x+xC​ and its derivative f′(x)=1−Cx2f'(x) = 1 - \frac{C}{x^2}f′(x)=1−x2C​, with the constraint that CCC is a non-zero constant.

Step 3: Evaluate Each Option

A: lim⁡x→0+f′(1x)=1\mathop {\lim }\limits_{x \to {0^ + }} f'\left( {{1 \over x}} \right) = 1x→0+lim​f′(x1​)=1

First, find the expression for f′(1x)f'(\frac{1}{x})f′(x1​): f′(1x)=1−C(1/x)2=1−Cx2f'\left( \frac{1}{x} \right) = 1 - \frac{C}{(1/x)^2} = 1 - Cx^2f′(x1​)=1−(1/x)2C​=1−Cx2 Now, we take the limit as x→0+x \to 0^+x→0+: lim⁡x→0+f′(1x)=lim⁡x→0+(1−Cx2)=1−C(0)=1\mathop {\lim }\limits_{x \to {0^ + }} f'\left( {{1 \over x}} \right) = \mathop {\lim }\limits_{x \to {0^ + }} (1 - Cx^2) = 1 - C(0) = 1x→0+lim​f′(x1​)=x→0+lim​(1−Cx2)=1−C(0)=1 This limit is 1, regardless of the value of CCC. So, Option A is correct.

B: lim⁡x→0+xf(1x)=2\mathop {\lim }\limits_{x \to {0^ + }} xf\left( {{1 \over x}} \right) = 2x→0+lim​xf(x1​)=2

First, find the expression for xf(1x)xf(\frac{1}{x})xf(x1​): f(1x)=1x+C1/x=1x+Cxf\left( \frac{1}{x} \right) = \frac{1}{x} + \frac{C}{1/x} = \frac{1}{x} + Cxf(x1​)=x1​+1/xC​=x1​+Cx xf(1x)=x(1x+Cx)=1+Cx2x f\left( \frac{1}{x} \right) = x \left( \frac{1}{x} + Cx \right) = 1 + Cx^2xf(x1​)=x(x1​+Cx)=1+Cx2 Now, we take the limit as x→0+x \to 0^+x→0+: lim⁡x→0+xf(1x)=lim⁡x→0+(1+Cx2)=1+C(0)=1\mathop {\lim }\limits_{x \to {0^ + }} xf\left( {{1 \over x}} \right) = \mathop {\lim }\limits_{x \to {0^ + }} (1 + Cx^2) = 1 + C(0) = 1x→0+lim​xf(x1​)=x→0+lim​(1+Cx2)=1+C(0)=1 The option states the limit is 2. Our calculated limit is 1. So, Option B is incorrect.

C: lim⁡x→0+x2f′(x)=0\mathop {\lim }\limits_{x \to {0^ + }} {x^2}f'(x) = 0x→0+lim​x2f′(x)=0

First, find the expression for x2f′(x)x^2 f'(x)x2f′(x): x2f′(x)=x2(1−Cx2)=x2−Cx^2 f'(x) = x^2 \left( 1 - \frac{C}{x^2} \right) = x^2 - Cx2f′(x)=x2(1−x2C​)=x2−C Now, we take the limit as x→0+x \to 0^+x→0+: lim⁡x→0+x2f′(x)=lim⁡x→0+(x2−C)=0−C=−C\mathop {\lim }\limits_{x \to {0^ + }} {x^2}f'(x) = \mathop {\lim }\limits_{x \to {0^ + }} (x^2 - C) = 0 - C = -Cx→0+lim​x2f′(x)=x→0+lim​(x2−C)=0−C=−C From the initial condition, we know C≠0C \neq 0C=0. Therefore, the limit is −C≠0-C \neq 0−C=0. The option states the limit is 0. So, Option C is incorrect.

D: ∣f(x)∣≤2|f(x)| \le 2∣f(x)∣≤2 for all x∈(0,2)x \in (0,2)x∈(0,2)

This statement must be true for any function fff that satisfies the given conditions, which means for any C≠0C \neq 0C=0. Let's test this with a specific case. Let's choose a simple non-zero value for CCC, say C=1C=1C=1. This corresponds to an initial condition like f(1)=2f(1)=2f(1)=2, which satisfies f(1)≠1f(1) \neq 1f(1)=1. For C=1C=1C=1, the function is f(x)=x+1xf(x) = x + \frac{1}{x}f(x)=x+x1​. Let's check the value of f(x)f(x)f(x) for an xxx in the interval (0,2)(0,2)(0,2). For instance, let x=0.5x=0.5x=0.5. f(0.5)=0.5+10.5=0.5+2=2.5f(0.5) = 0.5 + \frac{1}{0.5} = 0.5 + 2 = 2.5f(0.5)=0.5+0.51​=0.5+2=2.5 ∣f(0.5)∣=2.5|f(0.5)| = 2.5∣f(0.5)∣=2.5, which is not less than or equal to 2. Since we found a counterexample, the statement is not true for all possible functions fff. So, Option D is incorrect.

Conclusion

Based on the analysis, only option A is correct. Options B, C, and D are incorrect.

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