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JEE Advanced 2021 Paper 1 · Mathematics · Area Under Curves · medium

The parabola y^2=4x and its normal at the point (4,4) enclose a bounded region. Its area is

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The parabola y2=4xy^2=4x and its normal at the point (4,4)(4,4) enclose a bounded region. Its area is
  1. A. 643\dfrac{64}{3}
  2. B. 2503\dfrac{250}{3}
  3. C. 1003\dfrac{100}{3}
  4. D. 1253\dfrac{125}{3}
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Answer: (D) 1253\dfrac{125}{3}

For y2=4xy^2=4x the point (4,4)(4,4) corresponds to the parameter t=2t=2 in (t2,2t)(t^2,2t), and the slope of the tangent there is dydx=2y=12\dfrac{dy}{dx}=\dfrac{2}{y}=\dfrac12. The normal has slope −2-2: y−4=−2(x−4)y-4=-2(x-4), i.e. x=6−y2x=6-\dfrac y2. Intersecting with x=y24x=\dfrac{y^2}{4} gives y2+2y−24=0y^2+2y-24=0, so y=4y=4 or y=−6y=-6 (the second meeting point is (9,−6)(9,-6)). Integrating with respect to yy, the area is ∫−64(6−y2−y24)dy=[6y−y24−y312]−64=1253\displaystyle\int_{-6}^{4}\left(6-\frac y2-\frac{y^2}{4}\right)dy=\left[6y-\frac{y^2}{4}-\frac{y^3}{12}\right]_{-6}^{4}=\frac{125}{3}. Integrating along xx would require splitting the region, which is why yy is the better variable.
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