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Differential Equations question

2021 · 16 Mar · Shift 1 · Q37
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  5. /2021 · 16 Mar · Shift 1 · Q37

Differential Equations question

2021 · 16 Mar · Shift 1 · Q37

JEE MainMathematicsDifferential EquationsNumerical+4 / −1
Let the curve y = y(x) be the solution of the differential equation, dydx{{dy} \over {dx}}dxdy​= 2(x + 1). If the numerical value of area bounded by the curve y = y(x) and x-axis is 483{{4\sqrt 8 } \over 3}348​​, then the value of y(1) is equal to ‾\underline{\hspace{2cm}}​.
Numerical answer
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Correct answer: 2

  1. Given differential equation

dydx=2(x+1)\frac{dy}{dx}=2(x+1)dxdy​=2(x+1)

Integrate with respect to xxx:

y=∫2(x+1) dx=x2+2x+Cy=\int 2(x+1)\,dx=x^2+2x+Cy=∫2(x+1)dx=x2+2x+C

So the family of curves is

y=x2+2x+C=(x+1)2+(C−1)y=x^2+2x+C=(x+1)^2+(C-1)y=x2+2x+C=(x+1)2+(C−1)

  1. Find where the curve meets the x-axis

For the area bounded by the curve and the xxx-axis to be finite, the parabola must intersect the xxx-axis at two points.

Set y=0y=0y=0:

x2+2x+C=0x^2+2x+C=0x2+2x+C=0

Equivalently,

y=(x+1)2−(1−C)y=(x+1)^2-(1-C)y=(x+1)2−(1−C)

Let

a2=1−C(a>0)a^2=1-C \qquad (a>0)a2=1−C(a>0)

Then

y=(x+1)2−a2y=(x+1)^2-a^2y=(x+1)2−a2

The roots are at

x=−1−a,x=−1+ax=-1-a,\quad x=-1+ax=−1−a,x=−1+a

Between these roots, y≤0y\le 0y≤0, so the bounded area with the xxx-axis is

A=∫−1−a−1+a∣y∣ dx=∫−1−a−1+a(a2−(x+1)2)dxA=\int_{-1-a}^{-1+a} |y|\,dx=\int_{-1-a}^{-1+a} \left(a^2-(x+1)^2\right)dxA=∫−1−a−1+a​∣y∣dx=∫−1−a−1+a​(a2−(x+1)2)dx

  1. Compute the area

Put u=x+1u=x+1u=x+1. Then limits become −a-a−a to aaa:

A=∫−aa(a2−u2) duA=\int_{-a}^{a} (a^2-u^2)\,duA=∫−aa​(a2−u2)du

A=[a2u−u33]−aaA=\left[a^2u-\frac{u^3}{3}\right]_{-a}^{a}A=[a2u−3u3​]−aa​

A=4a33A=\frac{4a^3}{3}A=34a3​

Given

4a33=483\frac{4a^3}{3}=\frac{4\sqrt{8}}{3}34a3​=348​​

So,

a3=8=22=(2)3a^3=\sqrt{8}=2\sqrt{2}=(\sqrt{2})^3a3=8​=22​=(2​)3

Hence,

a=2a=\sqrt{2}a=2​

  1. Find the constant CCC

Since

a2=1−Ca^2=1-Ca2=1−C

we get

2=1−C2=1-C2=1−C

C=−1C=-1C=−1

Therefore,

y=x2+2x−1y=x^2+2x-1y=x2+2x−1

  1. Compute y(1)y(1)y(1)

y(1)=12+2(1)−1=1+2−1=2y(1)=1^2+2(1)-1=1+2-1=2y(1)=12+2(1)−1=1+2−1=2

Final Answer

2\boxed{2}2​

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