Let f(x): R+ = R+ is an invertible function such that f'(x) > 0 and f"(x) > 0 and f"(x)>0 for all x belongs to x [1, 5]. if f(1) = 1 and f(5) = 5 and area under the curve y = f(x) on x axis from x = 1 to x = 5 is 8 sq. unit, then area bounded by y = f^-1(x) axis from x = 1 to x = 5 is