Not a standard inequality

Calculus Level 5

Let f ( x ) , g ( x ) f(x), g(x) be functions from reals to reals and differentiable over all reals, such that the following are non-negative for all real x x :

f ( x ) f(x)

g ( x ) g(x)

x g ( x ) x-g'(x)

f ( x ) + f ( x ) g ( x ) f'(x)+f(x)g'(x)

If M M is the maximum possible value of f ( 0 ) f ( 1 ) \frac{f(0)}{f(1)} over all such functions f , g f, g satisfying the above and that f ( 1 ) f(1) is non-zero, what is the largest integer not exceeding 1000 M 2 1000M^{2} ?


The answer is 2718.

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1 solution

Joel Tan
Oct 28, 2016

Consider h ( x ) = e g ( x ) f ( x ) h(x) = e^{g(x)} f(x) , then h ( x ) = e g ( x ) ( f ( x ) + f ( x ) g ( x ) ) 0 h'(x) = e^{g(x)}(f'(x) + f(x)g'(x)) \geq 0 for all real x x , so h ( x ) h(x) is non-decreasing.

Thus f ( 0 ) f ( 1 ) = h ( 0 ) h ( 1 ) e g ( 1 ) g ( 0 ) e g ( 1 ) g ( 0 ) \frac{f(0)}{f(1)} = \frac{h(0)}{h(1)} e^{g(1)-g(0)} \leq e^{g(1)-g(0)} since h h is non-decreasing.

But g ( 1 ) g ( 0 ) = 0 1 g ( x ) d x 0 1 x d x = 1 2 g(1)-g(0) = \int_{0}^{1} g'(x) dx \leq \int_{0}^{1} x dx = \frac{1}{2} since x g ( x ) x \geq g'(x) .

Hence M = e 0.5 M = e^{0.5} , and the answer is 1000 M 2 = 2718.28... 1000M^{2} = 2718.28...

The equality case is left to the reader. (Hint: we need g ( x ) = x g'(x) = x and h ( x ) h(x) is constant)

Awesome solution.

Considering h(x) was an amazing idea.How did you thought of it?

Harsh Shrivastava - 4 years, 7 months ago

@Harsh Shrivastava It's just like the method for solving linear first order differential equations, multiply by e e raised to some function :)

Joel Tan - 4 years, 7 months ago

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