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Definite Integration question

2021 · 31 Aug · Shift 1 · Q43
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Definite Integration question

2021 · 31 Aug · Shift 1 · Q43

JEE MainMathematicsDefinite IntegrationNumerical+4 / −1
If xϕ(x)=∫5x(3t2−2ϕ′(t))dtx\phi (x) = \int\limits_5^x {(3{t^2} - 2\phi '(t))dt}xϕ(x)=5∫x​(3t2−2ϕ′(t))dt, x > −-− 2, and ϕ\phiϕ(0) = 4, then ϕ\phiϕ(2) is ‾\underline{\hspace{2cm}}​.
Numerical answer
View written solutionFree

Correct answer: 4

  1. Given equation

We have

xϕ(x)=∫5x(3t2−2ϕ′(t)) dt,x>−2x\phi(x)=\int_5^x \left(3t^2-2\phi'(t)\right)\,dt, \qquad x>-2xϕ(x)=∫5x​(3t2−2ϕ′(t))dt,x>−2

and ϕ(0)=4.\phi(0)=4.ϕ(0)=4.

We need to find ϕ(2)\phi(2)ϕ(2).


  1. Differentiate both sides with respect to xxx

Using the product rule on the left:

ddx[xϕ(x)]=ϕ(x)+xϕ′(x).\frac{d}{dx}[x\phi(x)] = \phi(x)+x\phi'(x).dxd​[xϕ(x)]=ϕ(x)+xϕ′(x).

Using the Fundamental Theorem of Calculus on the right:

ddx(∫5x(3t2−2ϕ′(t))dt)=3x2−2ϕ′(x).\frac{d}{dx}\left(\int_5^x (3t^2-2\phi'(t))dt\right)=3x^2-2\phi'(x).dxd​(∫5x​(3t2−2ϕ′(t))dt)=3x2−2ϕ′(x).

So,

ϕ(x)+xϕ′(x)=3x2−2ϕ′(x).\phi(x)+x\phi'(x)=3x^2-2\phi'(x).ϕ(x)+xϕ′(x)=3x2−2ϕ′(x).

Rearrange:

(x+2)ϕ′(x)+ϕ(x)=3x2.(x+2)\phi'(x)+\phi(x)=3x^2.(x+2)ϕ′(x)+ϕ(x)=3x2.
  1. Recognize the left side as a derivative

Notice that

ddx[(x+2)ϕ(x)]=(x+2)ϕ′(x)+ϕ(x).\frac{d}{dx}[(x+2)\phi(x)] = (x+2)\phi'(x)+\phi(x).dxd​[(x+2)ϕ(x)]=(x+2)ϕ′(x)+ϕ(x).

Hence the differential equation becomes

ddx[(x+2)ϕ(x)]=3x2.\frac{d}{dx}[(x+2)\phi(x)] = 3x^2.dxd​[(x+2)ϕ(x)]=3x2.

Integrate:

(x+2)ϕ(x)=∫3x2 dx=x3+C.(x+2)\phi(x)=\int 3x^2\,dx = x^3+C.(x+2)ϕ(x)=∫3x2dx=x3+C.

Thus,

ϕ(x)=x3+Cx+2.\phi(x)=\frac{x^3+C}{x+2}.ϕ(x)=x+2x3+C​.
  1. Use the initial condition ϕ(0)=4\phi(0)=4ϕ(0)=4

Substitute x=0x=0x=0:

ϕ(0)=03+C0+2=C2=4.\phi(0)=\frac{0^3+C}{0+2}=\frac{C}{2}=4.ϕ(0)=0+203+C​=2C​=4.

So,

C=8.C=8.C=8.

Therefore,

ϕ(x)=x3+8x+2.\phi(x)=\frac{x^3+8}{x+2}.ϕ(x)=x+2x3+8​.

Factor the numerator:

x3+8=(x+2)(x2−2x+4).x^3+8=(x+2)(x^2-2x+4).x3+8=(x+2)(x2−2x+4).

Since x>−2x>-2x>−2, we can simplify:

ϕ(x)=x2−2x+4.\phi(x)=x^2-2x+4.ϕ(x)=x2−2x+4.
  1. Compute ϕ(2)\phi(2)ϕ(2)
ϕ(2)=22−2(2)+4=4−4+4=4.\phi(2)=2^2-2(2)+4=4-4+4=4.ϕ(2)=22−2(2)+4=4−4+4=4.
  1. Compare with stored answer

Derived answer: 444

Stored correct answer: 444

They agree.

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