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Area Under the Curves question

2009 · Shift 0 · Q34
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  5. /2009 · Shift 0 · Q34

Area Under the Curves question

2009 · Shift 0 · Q34

JEE MainMathematicsArea Under the CurvesMCQ+4 / −1
The area of the region bounded by the parabola (y−2)2=x−1,{\left( {y - 2} \right)^2} = x - 1,(y−2)2=x−1, the tangent of the parabola at the point (2,3)(2, 3)(2,3) and the xxx-axis is :
  1. A
    666
  2. B
    999
  3. C
    121212
  4. D
    333
View written solutionFree

Correct answer: B

  1. Given parabola

    (y−2)2=x−1(y-2)^2 = x-1(y−2)2=x−1

    Rewrite in terms of xxx:

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

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

    First verify the point lies on the parabola:

    (3−2)2=2−1=1(3-2)^2 = 2-1 = 1(3−2)2=2−1=1

    so (2,3)(2,3)(2,3) is on the curve.

    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 dydx=12(y−2)\frac{dy}{dx} = \frac{1}{2(y-2)}dxdy​=2(y−2)1​

    At (2,3)(2,3)(2,3), we have y−2=1y-2=1y−2=1, hence

    dydx∣(2,3)=12\left.\frac{dy}{dx}\right|_{(2,3)} = \frac12dxdy​​(2,3)​=21​

    Equation of tangent through (2,3)(2,3)(2,3):

    y−3=12(x−2)y-3 = \frac12(x-2)y−3=21​(x−2)

    2y−6=x−22y-6 = x-22y−6=x−2 x=2y−4x = 2y-4x=2y−4

  3. Find intersections with the xxx-axis

    The xxx-axis is y=0y=0y=0.

    • With the parabola: (0−2)2=x−1(0-2)^2 = x-1(0−2)2=x−1 4=x−14 = x-14=x−1 x=5x=5x=5 So point is (5,0)(5,0)(5,0).

    • With the tangent: 0−3=12(x−2)0-3 = \frac12(x-2)0−3=21​(x−2) −3=12(x−2)-3 = \frac12(x-2)−3=21​(x−2) −6=x−2-6 = x-2−6=x−2 x=−4x=-4x=−4 So point is (−4,0)(-4,0)(−4,0).

  4. Understand the bounded region

    The closed region is bounded by:

    • the tangent from (−4,0)(-4,0)(−4,0) to (2,3)(2,3)(2,3),
    • the parabola from (2,3)(2,3)(2,3) to (5,0)(5,0)(5,0),
    • the xxx-axis from (5,0)(5,0)(5,0) back to (−4,0)(-4,0)(−4,0).

    Since both curves are conveniently written as xxx in terms of yyy, integrate with respect to yyy from y=0y=0y=0 to y=3y=3y=3.

    For a horizontal strip:

    • right boundary: parabola x=(y−2)2+1=y2−4y+5x = (y-2)^2+1 = y^2-4y+5x=(y−2)2+1=y2−4y+5
    • left boundary: tangent x=2y−4x = 2y-4x=2y−4

    So area is

    A=∫03[(y2−4y+5)−(2y−4)] dyA = \int_0^3 \Big[(y^2-4y+5) - (2y-4)\Big] \, dyA=∫03​[(y2−4y+5)−(2y−4)]dy

    A=∫03(y2−6y+9) dyA = \int_0^3 (y^2 - 6y + 9)\,dyA=∫03​(y2−6y+9)dy

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

  5. Evaluate the integral

    A=[y33−3y2+9y]03A = \left[\frac{y^3}{3} - 3y^2 + 9y\right]_0^3A=[3y3​−3y2+9y]03​

    At y=3y=3y=3: 273−3(9)+27=9−27+27=9\frac{27}{3} - 3(9) + 27 = 9 - 27 + 27 = 9327​−3(9)+27=9−27+27=9

    At y=0y=0y=0: 000

    Therefore,

    A=9A = 9A=9

  6. Check options

    The correct option is:

    B: 9\boxed{\text{B: }9}B: 9​

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