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JEE Advanced 2018 · Paper 1, Mathematics question 14

A farmer F1F_1 has a land in the shape of a triangle with vertices at P(0,0)P(0, 0), Q(1,1)Q(1, 1) and R(2,0)R(2, 0). From this land, a neighbouring farmer F2F_2 takes away the region which lies between the side PQPQ and a curve of the form y=xny = x^n (n>1)(n > 1). If the area of the region taken away by the farmer F2F_2 is exactly 30% of the area of △PQR\triangle PQR, then the value of nn is _____ .

Answer and worked solution

Official answer: 4

Triangle. The area of △PQR\triangle PQR is 12×2×1=1\tfrac12\times2\times1=1.

Region taken. Side PQPQ is y=xy=x for 0≤x≤10\le x\le1. The curve y=xny=x^n lies below it there:
∫01(x−xn) dx=12−1n+1=0.3  ⟹  1n+1=0.2  ⟹  n=4.\int_0^1(x-x^n)\,dx=\frac12-\frac1{n+1}=0.3\implies\frac1{n+1}=0.2\implies n=4.

Answer: 4