Let be functions from reals to reals and differentiable over all reals, such that the following are non-negative for all real :
If is the maximum possible value of over all such functions satisfying the above and that is non-zero, what is the largest integer not exceeding ?
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Consider h ( x ) = e g ( x ) f ( x ) , then h ′ ( x ) = e g ( x ) ( f ′ ( x ) + f ( x ) g ′ ( x ) ) ≥ 0 for all real x , so h ( x ) is non-decreasing.
Thus f ( 1 ) f ( 0 ) = h ( 1 ) h ( 0 ) e g ( 1 ) − g ( 0 ) ≤ e g ( 1 ) − g ( 0 ) since h is non-decreasing.
But g ( 1 ) − g ( 0 ) = ∫ 0 1 g ′ ( x ) d x ≤ ∫ 0 1 x d x = 2 1 since x ≥ g ′ ( x ) .
Hence M = e 0 . 5 , and the answer is 1 0 0 0 M 2 = 2 7 1 8 . 2 8 . . .
The equality case is left to the reader. (Hint: we need g ′ ( x ) = x and h ( x ) is constant)