Simple Isn't it?

Calculus Level 2

True or False:

e π e^{π} is greater than π e π^{e} .

True False

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1 solution

1.-Consider the function f ( x ) = x 1 x = e 1 x ln x x > 0 , x R f(x) = x^{\frac{1}{x}} = e^{\frac{1}{x}\ln x} \text{ } \forall x > 0,x \in \mathbb{R} . Differentiating gives f ( x ) = x 1 x 1 x 2 ( 1 l n x ) f ( e ) = 0 and f ( x ) > 0 x < e , f ( x ) < 0 x > e f '(x) = x^{\frac{1}{x}} \cdot \frac{1}{x^2}(1-lnx) \Rightarrow f '(e) = 0 \text{ and } f '(x) > 0 \text{ }\forall x < e, f '(x) < 0 \text{ } \forall x > e , so the function attains its global maximum at x=e.

Thus e 1 e π 1 π ( e 1 e ) e π = e π ( π 1 π ) e π = π e e^{\frac{1}{e}} \geq π^{\frac{1}{π}} \Rightarrow (e^{\frac{1}{e}})^{e\pi} = e^{\pi} \geq (π^{\frac{1}{π}})^{e \pi} = \pi^{e} and the inequality is clearly strict.

or

2.-If you know Taylor expansion: e x = 1 + x + x 2 2 ! + . . . e^{x} = 1 + x + \frac{x^2}{2!} +...\Rightarrow

e x > 1 + x , x > 0 e^{x} >1 + x,∀x>0

Then set x = π e 1 > 0 x = \frac{π}{e} -1 >0

We get e π e 1 > 1 + π e 1 e π e e > π e e π e > π e π > π e e^{\frac{π}{e}-1} > 1 + \frac{π}{e}-1 ⇔ \frac {e^{\frac {π}{e}}}{e} > \frac {π}{e} ⇔ e^{\frac{π}{e}} >π ⇔ e^π > π^e

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