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← JEE Advanced 2019 · Paper 1
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JEE Advanced 2019 · Paper 1, Mathematics question 4

The area of the region {(x,y):xy≤8, 1≤y≤x2}\{(x,y) : xy \le 8,\ 1 \le y \le x^2\} is

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Answer and worked solution

Official answer: A

Take x>0x>0 (the region intended). For each y≥1y\ge1 the region runs from x=yx=\sqrt y to x=8yx=\frac8y. It is non-empty while y≤8y\sqrt y\le\frac8y, i.e. y≤4y\le4.

Area=∫14(8y−y)dy=8ln⁡4−23(8−1)=16log⁡e2−143.\text{Area}=\int_1^4\left(\frac8y-\sqrt y\right)dy=8\ln4-\frac23(8-1)=16\log_e2-\frac{14}3.
(A)