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Area Under the Curves question

2021 · 27 Aug · Shift 2 · Q32
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Area Under the Curves question

2021 · 27 Aug · Shift 2 · Q32

JEE MainMathematicsArea Under the CurvesMCQ+4 / −1
The area of the region bounded by the parabola (y −-− 2)2 = (x −-− 1), the tangent to it at the point whose ordinate is 3 and the x-axis is :
  1. A
    9
  2. B
    10
  3. C
    4
  4. D
    6
View written solutionFree

Correct answer: A

  1. Write the parabola in a convenient form

Given: (y−2)2=x−1(y-2)^2 = x-1(y−2)2=x−1 So, x=(y−2)2+1x = (y-2)^2 + 1x=(y−2)2+1 This is a right-opening parabola with vertex at (1,2)(1,2)(1,2).

  1. Find the point on the parabola whose ordinate is 3

Ordinate means the yyy-coordinate. So put y=3y=3y=3: x=(3−2)2+1=1+1=2x = (3-2)^2 + 1 = 1+1 = 2x=(3−2)2+1=1+1=2 Hence the point is: (2,3)(2,3)(2,3)

  1. Find the tangent at (2,3)(2,3)(2,3)

Differentiate implicitly: (y−2)2=x−1(y-2)^2 = x-1(y−2)2=x−1 2(y−2)dydx=12(y-2)\frac{dy}{dx} = 12(y−2)dxdy​=1 So, dydx=12(y−2)\frac{dy}{dx} = \frac{1}{2(y-2)}dxdy​=2(y−2)1​ At y=3y=3y=3, dydx=12\frac{dy}{dx} = \frac{1}{2}dxdy​=21​ Thus tangent equation at (2,3)(2,3)(2,3) is: y−3=12(x−2)y-3 = \frac{1}{2}(x-2)y−3=21​(x−2) 2y−6=x−22y-6 = x-22y−6=x−2 x=2y−4x = 2y-4x=2y−4

  1. Understand the bounded region

The region is bounded by:

  • the parabola: x=(y−2)2+1x=(y-2)^2+1x=(y−2)2+1
  • the tangent: x=2y−4x=2y-4x=2y−4
  • the xxx-axis: y=0y=0y=0

We should find where the tangent and parabola touch: that is at y=3y=3y=3. So the enclosed region runs from y=0y=0y=0 to y=3y=3y=3.

Now compare left/right curves in this interval.

For example at y=1y=1y=1:

  • parabola: x=(1−2)2+1=2x=(1-2)^2+1=2x=(1−2)2+1=2
  • tangent: x=2(1)−4=−2x=2(1)-4=-2x=2(1)−4=−2

So parabola is on the right and tangent is on the left.

Therefore area is: A=∫03[((y−2)2+1)−(2y−4)]dyA = \int_0^3 \left[\big((y-2)^2+1\big) - (2y-4)\right]dyA=∫03​[((y−2)2+1)−(2y−4)]dy

  1. Simplify the integrand

(y−2)2+1−(2y−4)=(y2−4y+4)+1−2y+4(y-2)^2+1-(2y-4) = (y^2-4y+4)+1-2y+4(y−2)2+1−(2y−4)=(y2−4y+4)+1−2y+4 =y2−6y+9= y^2 - 6y + 9=y2−6y+9 =(y−3)2= (y-3)^2=(y−3)2

So, A=∫03(y−3)2 dyA = \int_0^3 (y-3)^2 \, dyA=∫03​(y−3)2dy

  1. Evaluate the integral

∫(y−3)2dy=(y−3)33\int (y-3)^2 dy = \frac{(y-3)^3}{3}∫(y−3)2dy=3(y−3)3​ Thus, A=[(y−3)33]03A = \left[\frac{(y-3)^3}{3}\right]_0^3A=[3(y−3)3​]03​ =033−(−3)33= \frac{0^3}{3} - \frac{(-3)^3}{3}=303​−3(−3)3​ =0−(−273)= 0 - \left(\frac{-27}{3}\right)=0−(3−27​) =9= 9=9

  1. Final answer

The required area is: 9\boxed{9}9​

So the correct option is A.

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