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JEE Advanced 2020 Paper 1 · Mathematics · Area Under Curves · hard

The area of the region \(x,y):x^2+y^2≤ 8, y^2≥ 2x\ is

Single correct option

The area of the region {(x,y):x2+y2≤8, y2≥2x}\{(x,y):x^{2}+y^{2}\le 8,\ y^{2}\ge 2x\} is
  1. A. 6π+436\pi+\dfrac43
  2. B. 2π+432\pi+\dfrac43
  3. C. 4π−434\pi-\dfrac43
  4. D. 6π−436\pi-\dfrac43
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Answer: (D) 6π−436\pi-\dfrac43

The circle has area 8π8\pi. The parabola y2=2xy^2=2x meets the circle where x2+2x=8x^2+2x=8, i.e. at x=2x=2, y=±2y=\pm2. The part of the disc lying inside the parabola (y2≤2xy^2\le2x) is bounded by the parabola for 0≤x≤20\le x\le2 and by the circle for 2≤x≤222\le x\le2\sqrt2. Its area is 2[∫022x dx+∫2228−x2 dx]2\left[\int_0^2\sqrt{2x}\,dx+\int_2^{2\sqrt2}\sqrt{8-x^2}\,dx\right]. The first integral equals 8/38/3. The second equals [x28−x2+4sin⁡−1x22]222=2π−(2+π)=π−2\left[\tfrac x2\sqrt{8-x^2}+4\sin^{-1}\tfrac{x}{2\sqrt2}\right]_2^{2\sqrt2}=2\pi-(2+\pi)=\pi-2. So the inside part is 2(π+23)=2π+432(\pi+\tfrac23)=2\pi+\tfrac43, and the required area is 8π−2π−43=6π−438\pi-2\pi-\tfrac43=6\pi-\tfrac43. A numerical integration agrees (≈17.516\approx17.516).
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