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Area Under the Curves question

2025 · 22 Jan · Shift 2 · Q37
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  5. /2025 · 22 Jan · Shift 2 · Q37

Area Under the Curves question

2025 · 22 Jan · Shift 2 · Q37

JEE MainMathematicsArea Under the CurvesMCQ+4 / −1
The area of the region enclosed by the curves y=x2−4x+4y=x^2-4 x+4y=x2−4x+4 and y2=16−8xy^2=16-8 xy2=16−8x is :
  1. A
    83\frac{8}{3}38​
  2. B
    555
  3. C
    888
  4. D
    43\frac{4}{3}34​
View written solutionFree

Correct answer: A

  1. Write the curves in simpler form

The given curves are y=x2−4x+4=(x−2)2y=x^2-4x+4=(x-2)^2y=x2−4x+4=(x−2)2 and y2=16−8x.y^2=16-8x.y2=16−8x.

Rewrite the second curve in terms of xxx: 8x=16−y28x=16-y^28x=16−y2 x=2−y28.x=2-\frac{y^2}{8}.x=2−8y2​.

Also, from y=(x−2)2,y=(x-2)^2,y=(x−2)2, we can write x=2±y.x=2\pm \sqrt{y}.x=2±y​. Since this parabola has y≥0y\ge 0y≥0, only the upper half is relevant.

  1. Find the points of intersection

From y=(x−2)2y=(x-2)^2y=(x−2)2 and y2=16−8x,y^2=16-8x,y2=16−8x, substitute x=2±yx=2\pm\sqrt yx=2±y​ into the second form. But an easier way is to use the second curve as x=2−y28x=2-\frac{y^2}{8}x=2−8y2​ and compare with the left branch of the first curve: x=2−y.x=2-\sqrt y.x=2−y​.

At intersection, 2−y=2−y282-\sqrt y=2-\frac{y^2}{8}2−y​=2−8y2​ y=y28.\sqrt y=\frac{y^2}{8}.y​=8y2​. Let t=yt=\sqrt yt=y​ so that y=t2y=t^2y=t2. Then t=t48t=\frac{t^4}{8}t=8t4​ t4−8t=0t^4-8t=0t4−8t=0 t(t3−8)=0t(t^3-8)=0t(t3−8)=0 t=0 or t=2.t=0 \text{ or } t=2.t=0 or t=2. Thus, y=0 or y=4.y=0 \text{ or } y=4.y=0 or y=4.

Now find corresponding xxx:

  • For y=0y=0y=0, from y=(x−2)2y=(x-2)^2y=(x−2)2, we get x=2x=2x=2.
  • For y=4y=4y=4, from y=(x−2)2y=(x-2)^2y=(x−2)2, we get (x−2)2=4(x-2)^2=4(x−2)2=4, so x=0x=0x=0 or 444. But on the parabola x=2−y28=2−2=0x=2-\frac{y^2}{8}=2-2=0x=2−8y2​=2−2=0, so the common point is (0,4)(0,4)(0,4).

Hence the curves intersect at (2,0)and(0,4).(2,0) \quad \text{and} \quad (0,4).(2,0)and(0,4).

  1. Identify the enclosed region

For 0≤y≤40\le y\le 40≤y≤4:

  • Left boundary comes from y2=16−8xy^2=16-8xy2=16−8x: x=2−y28.x=2-\frac{y^2}{8}.x=2−8y2​.
  • Right boundary comes from the left branch of y=(x−2)2y=(x-2)^2y=(x−2)2: x=2−y.x=2-\sqrt y.x=2−y​.

So area is A=∫04[(2−y)−(2−y28)]dy.A=\int_0^4 \left[\left(2-\sqrt y\right)-\left(2-\frac{y^2}{8}\right)\right]dy.A=∫04​[(2−y​)−(2−8y2​)]dy.

Simplify: A=∫04(y28−y)dy.A=\int_0^4 \left(\frac{y^2}{8}-\sqrt y\right)dy.A=∫04​(8y2​−y​)dy.

But check sign at, say, y=1y=1y=1: 2−18=1.875,2−1=1.2-\frac{1}{8}=1.875, \quad 2-1=1.2−81​=1.875,2−1=1. So actually the curve x=2−y28x=2-\frac{y^2}{8}x=2−8y2​ lies to the right of x=2−yx=2-\sqrt yx=2−y​.

Therefore, A=∫04[(2−y28)−(2−y)]dyA=\int_0^4 \left[\left(2-\frac{y^2}{8}\right)-\left(2-\sqrt y\right)\right]dyA=∫04​[(2−8y2​)−(2−y​)]dy =∫04(y−y28)dy.=\int_0^4 \left(\sqrt y-\frac{y^2}{8}\right)dy.=∫04​(y​−8y2​)dy.

  1. Evaluate the integral

A=∫04y1/2 dy−18∫04y2 dy.A=\int_0^4 y^{1/2}\,dy-\frac{1}{8}\int_0^4 y^2\,dy.A=∫04​y1/2dy−81​∫04​y2dy.

Now, ∫04y1/2 dy=[23y3/2]04=23⋅8=163.\int_0^4 y^{1/2}\,dy=\left[\frac{2}{3}y^{3/2}\right]_0^4=\frac{2}{3}\cdot 8=\frac{16}{3}.∫04​y1/2dy=[32​y3/2]04​=32​⋅8=316​.

And 18∫04y2 dy=18[y33]04=18⋅643=83.\frac{1}{8}\int_0^4 y^2\,dy=\frac{1}{8}\left[\frac{y^3}{3}\right]_0^4=\frac{1}{8}\cdot\frac{64}{3}=\frac{8}{3}.81​∫04​y2dy=81​[3y3​]04​=81​⋅364​=38​.

Thus, A=163−83=83.A=\frac{16}{3}-\frac{8}{3}=\frac{8}{3}.A=316​−38​=38​.

  1. Compare with options

The area is 83\boxed{\frac{8}{3}}38​​ which corresponds to Option A.

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