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Differential Equations question

2011 · Shift 1 · Q46
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  5. /2011 · Shift 1 · Q46

Differential Equations question

2011 · Shift 1 · Q46

JEE AdvancedMathematicsDifferential EquationsNumerical+3 / −1
Let f:[1,∞)→[2,∞)f:[1,\infty ) \to [2,\infty )f:[1,∞)→[2,∞) be a differentiable function such that f(1)=2f(1) = 2f(1)=2. If 6∫1xf(t)dt=3xf(x)−x3−56\int\limits_1^x {f(t)dt = 3xf(x) - {x^3} - 5}61∫x​f(t)dt=3xf(x)−x3−5 for all x≥1x \ge 1x≥1, then the value of f(2) is ‾\underline{\hspace{2cm}}​.
Numerical answer
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Correct answer: 6

The user wants me to solve for the value of f(2) given an integral equation and an initial condition.

Step-by-step Solution:

  1. Start with the given integral equation: We are given the equation for all x≥1x \ge 1x≥1: 6∫1xf(t)dt=3xf(x)−x3−56\int\limits_1^x {f(t)dt = 3xf(x) - {x^3} - 5}61∫x​f(t)dt=3xf(x)−x3−5

  2. Differentiate both sides with respect to x: To eliminate the integral, we can differentiate both sides of the equation with respect to xxx. We will use the Leibniz rule (Fundamental Theorem of Calculus, Part 1) for the left side and the product rule for the right side.

    • Left Hand Side (LHS): ddx(6∫1xf(t)dt)=6f(x)\frac{d}{{dx}}\left( {6\int\limits_1^x {f(t)dt} } \right) = 6f(x)dxd​(61∫x​f(t)dt)=6f(x)

    • Right Hand Side (RHS): ddx(3xf(x)−x3−5)\frac{d}{{dx}}\left( {3xf(x) - {x^3} - 5} \right)dxd​(3xf(x)−x3−5) Using the product rule for the term 3xf(x)3xf(x)3xf(x): ddx(3xf(x))=(ddx(3x))f(x)+3x(ddxf(x))=3f(x)+3xf′(x)\frac{d}{{dx}}(3xf(x)) = \left( {\frac{d}{{dx}}(3x)} \right)f(x) + 3x\left( {\frac{d}{{dx}}f(x)} \right) = 3f(x) + 3xf'(x)dxd​(3xf(x))=(dxd​(3x))f(x)+3x(dxd​f(x))=3f(x)+3xf′(x) The derivative of the entire RHS is: 3f(x)+3xf′(x)−3x2−03f(x) + 3xf'(x) - 3{x^2} - 03f(x)+3xf′(x)−3x2−0

  3. Form the differential equation: Equating the derivatives of the LHS and RHS: 6f(x)=3f(x)+3xf′(x)−3x26f(x) = 3f(x) + 3xf'(x) - 3{x^2}6f(x)=3f(x)+3xf′(x)−3x2 Rearranging the terms to simplify: 6f(x)−3f(x)=3xf′(x)−3x26f(x) - 3f(x) = 3xf'(x) - 3{x^2}6f(x)−3f(x)=3xf′(x)−3x2 3f(x)=3xf′(x)−3x23f(x) = 3xf'(x) - 3{x^2}3f(x)=3xf′(x)−3x2 Dividing the entire equation by 3: f(x)=xf′(x)−x2f(x) = xf'(x) - {x^2}f(x)=xf′(x)−x2 Let y=f(x)y = f(x)y=f(x), so f′(x)=dydxf'(x) = \frac{{dy}}{{dx}}f′(x)=dxdy​. The equation becomes: y=xdydx−x2y = x\frac{{dy}}{{dx}} - {x^2}y=xdxdy​−x2 Rearranging this into the standard form of a linear first-order differential equation, dydx+P(x)y=Q(x)\frac{{dy}}{{dx}} + P(x)y = Q(x)dxdy​+P(x)y=Q(x): xdydx−y=x2x\frac{{dy}}{{dx}} - y = {x^2}xdxdy​−y=x2 dydx−1xy=x\frac{{dy}}{{dx}} - \frac{1}{x}y = xdxdy​−x1​y=x

  4. Solve the linear differential equation: This is a linear differential equation with P(x)=−1xP(x) = -\frac{1}{x}P(x)=−x1​ and Q(x)=xQ(x) = xQ(x)=x. First, find the integrating factor (I.F.): I.F.=e∫P(x)dx=e∫−1xdx=e−ln⁡∣x∣=eln⁡(∣x∣−1)=1∣x∣I.F. = {e^{\int {P(x)dx} }} = {e^{\int { - \frac{1}{x}dx} }} = {e^{ - \ln |x|}} = {e^{\ln ({|x|^{ - 1}})}} = \frac{1}{{|x|}}I.F.=e∫P(x)dx=e∫−x1​dx=e−ln∣x∣=eln(∣x∣−1)=∣x∣1​ Since we are given that x≥1x \ge 1x≥1, we have ∣x∣=x|x| = x∣x∣=x. So, the integrating factor is: I.F.=1xI.F. = \frac{1}{x}I.F.=x1​ The solution to the differential equation is given by: y⋅(I.F.)=∫Q(x)⋅(I.F.)dx+Cy \cdot (I.F.) = \int {Q(x) \cdot (I.F.)dx} + Cy⋅(I.F.)=∫Q(x)⋅(I.F.)dx+C y⋅1x=∫x⋅1xdx+Cy \cdot \frac{1}{x} = \int {x \cdot \frac{1}{x}dx} + Cy⋅x1​=∫x⋅x1​dx+C yx=∫1dx+C\frac{y}{x} = \int {1dx} + Cxy​=∫1dx+C yx=x+C\frac{y}{x} = x + Cxy​=x+C y=x2+Cxy = {x^2} + Cxy=x2+Cx So, the function is f(x)=x2+Cxf(x) = {x^2} + Cxf(x)=x2+Cx.

  5. Use the initial condition to find C: We are given the condition f(1)=2f(1) = 2f(1)=2. Substitute x=1x = 1x=1 into the expression for f(x)f(x)f(x): f(1)=12+C(1)f(1) = {1^2} + C(1)f(1)=12+C(1) 2=1+C2 = 1 + C2=1+C C=1C = 1C=1

  6. Determine the specific function f(x): Substituting C=1C=1C=1 back into the general solution, we get: f(x)=x2+xf(x) = {x^2} + xf(x)=x2+x

  7. Calculate the value of f(2): Now, we can find the value of f(2)f(2)f(2) by substituting x=2x=2x=2 into the function: f(2)=22+2f(2) = {2^2} + 2f(2)=22+2 f(2)=4+2f(2) = 4 + 2f(2)=4+2 f(2)=6f(2) = 6f(2)=6

Final Answer:

The value of f(2) is 6.

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