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JEE Advanced 2020 Paper 2 · Mathematics · Area Under Curves · medium

Find the area of the region lying inside the circle x^2+y^2=8 and on or above the parabola y=(x^2/(2).

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Find the area of the region lying inside the circle x2+y2=8x^{2}+y^{2}=8 and on or above the parabola y=x22y=\dfrac{x^{2}}{2}. Give the answer correct to two decimal places.
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Answer: 7.62

Eliminate yy: x2+x44=8x^2+\frac{x^4}{4}=8 gives x4+4x2−32=0x^4+4x^2-32=0, so x2=4x^2=4 and the curves meet at (±2,2)(\pm2,2). Between x=−2x=-2 and x=2x=2 the upper boundary is the upper semicircle y=8−x2y=\sqrt{8-x^2} and the lower boundary is the parabola. Area =∫−22(8−x2−x22)dx=\int_{-2}^{2}\left(\sqrt{8-x^2}-\frac{x^2}{2}\right)dx. Using ∫8−x2 dx=x28−x2+4sin⁡−1x22\int\sqrt{8-x^2}\,dx=\frac x2\sqrt{8-x^2}+4\sin^{-1}\frac{x}{2\sqrt2}, the circle part from −2-2 to 22 equals 2(2+4⋅π4)=4+2π2\left(2+4\cdot\frac\pi4\right)=4+2\pi. The parabola part is 12⋅163=83\frac12\cdot\frac{16}{3}=\frac83. So the area is 4+2π−83=43+2π≈7.61654+2\pi-\frac83=\frac43+2\pi\approx7.6165, i.e. 7.627.62. The two intersection points (±2,2)(\pm2,2) lie on the circle since 4+4=84+4=8, and the parabola is below the upper semicircle between them, which justifies the choice of upper and lower curves in the integral. The answer is rounded to two decimals.
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