Which one is greater?

Calculus Level 2

e π or π e \Large \color{#3D99F6}{e}^{\color{#D61F06}{\pi}} \quad \text{or} \quad \color{#D61F06}{\pi}^{\color{#3D99F6}{e}}

Which of the above numbers is larger?

They are equal Not enough information π e π^{e} e π e^{π}

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8 solutions

Jessica Wang
Nov 24, 2015

e x = 1 + x + x 2 2 ! + . . . \because e^{x}=1+x+\frac{x^{2}}{2!}+... e x > 1 + x . \therefore e^{x}> 1+x. Substitute in x = π e 1 x=\frac{\pi }{e}-1 , giving e π e 1 > π e e^{\frac{\pi}{e}-1}> \frac{\pi }{e} e π e > π \Rightarrow e^{\frac{\pi}{e}}> \pi e π > π e . \Rightarrow e^{\pi}> \pi ^{e}.

since e^x > 1+x we can get a beter approximation posing x=π/e :

e^(π/e) > 1+ π/e leads to e^(e+π) > (e+π)^e

Sam Lee - 5 years, 6 months ago

The first line is always true. The second line is true if x x is positive. What if x x is negative?

Jack Cornish - 5 years, 6 months ago

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It works with any x, even negative ones. It's a series taught in most Calculus 2 courses.

William Clukey - 5 years, 6 months ago

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Yes, that is true. But it is more difficult to conclude that the negative case is true, i.e. you can't just conclude it by the algebra given in the solution I referred to.

Jack Cornish - 5 years, 6 months ago

since pi is greater than e pi/e -l is positive

Chinmay Krishna - 5 years, 6 months ago

It would be e x 1 + x e^x\geq 1+x , as when x = 0 x=0 , it gives 1 1 at the left side and... 1 1 at the right one. Not that it affects your reasoning.

David Molano - 5 years, 6 months ago

Elegant! I like it.

Chia Stockwell - 5 years, 6 months ago

Consider a function f(x) = x 1 x x^{ \dfrac{1}{x} }

Differentiating with respect to x,

f '(x) = x 1 x × [ 1 x 2 l n ( x ) x 2 ] x^{ \dfrac{1}{x} } \times [ \dfrac{1}{x^2} - \dfrac{ln(x)}{x^2} ]

When f(x) is decreasing f '(x) < 0,

Since x 1 x x^{ \dfrac{1}{x} } & 1 x 2 \dfrac{1}{x^2} are greater than 0, it implies 1 l n ( x ) < 0 1 - ln(x) < 0

l n ( x ) > 1 ln(x) > 1

x > e x > e

Hence, f(x) is decreasing in the region, x > e.

By definition of decreasing function as,

π > e \pi > e

f ( π ) < f ( e ) f( \pi ) < f(e)

π 1 π < e 1 e \pi^\dfrac{1}{\pi} < e^\dfrac{1}{e}

Raising to π × e \pi \times e on both sides,

π e < e π \boxed{ \pi^{e} < e^{\pi}}

Fantastic.

Պոոռնապռագնյա ՊՌ - 5 years, 6 months ago

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Thanks, but it's just basic use of differentiation.

A Former Brilliant Member - 5 years, 6 months ago

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Mohammed azad is actually right. Please explain how 1/(x^x) is positive?

Ananya Prakash - 5 years, 6 months ago

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@Ananya Prakash The function isn't defined on x < 0. And it is always greater than 0 for x > 0.

A Former Brilliant Member - 5 years, 6 months ago

say something

Ujjal Purkayastha - 5 years, 6 months ago

How is it that 1/(x)^(x) is greater than 0?! To further assert my point: 1/(-1)^(-1) is -1 which is clearly less than 0. And something else The derivative for f(x) = 1/x^x is f'(x) = -x^(-x) * (1+ln(x))

Mohamed Azab - 5 years, 6 months ago

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The function isn't defined for x < 0

A Former Brilliant Member - 5 years, 6 months ago

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if I'm not mistaken, undefined means not solvable. in the instant example, it clearly was solved - and therefore is clearly defined.

Marc Brothers - 5 years, 6 months ago

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@Marc Brothers @Marc Brothers ! For Better understanding

By undefined, I mean that the function doesn't have real values at those x. Eg : y = x \sqrt{x} is not defined for x <0. For the function I specified it is undefined is cause at all points between 2 negative integers ( 1 ) s o m e t h i n g f r a c t i o n a l (-1)^{something fractional} is not defined.

A Former Brilliant Member - 5 years, 6 months ago

What about the derivative?! It's incorrect as I said. And what exactly did you prove near the end?! That 1/(pi^pi) < 1/(e^e) Well, one doesn't need to differentiate anything to come up with this result because 1/x^x is always smaller than 1/y^y if x > y for x, y >= 1 because a positive number greater than one raised to itself is always smaller than another number that is greater than it when raised to itself. The same with 2^2 < 3^3 and I fail to see how you used this finding to come with your final answer. And about your differentiation, the correct answer would be that For x>0, 1/x^x is decreasing as x>1/e, and since both pi and e are greater than 1/e, you didn't have any trouble proving that 1/pi^pi < 1/e^e which didn't need proving and I can't see how it led you to your final conclusion. To prove my point about the differentiatipn, you said that for x>0, the function is decreasing on x>e, then try 1/2^2 < 1/1^1 Here the function is decreasing for x less than e.

Mohamed Azab - 5 years, 6 months ago

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@Mohamed Azab The function isn't 1 x x \dfrac{1}{x^{x}} , it is x 1 x x^{\dfrac{1}{x}} , which are two completely different functions. So half of your comment doesn't make sense whatsoever. The differentiation I did is correct, you could check it out. Thus, the function is decreasing for x > e, which is exactly what I wrote.

A Former Brilliant Member - 5 years, 6 months ago

Well, why would you go to the extra effort of using a differentiation for such a simple question? Both numbers are constants. Up to 20 decimal places, these are the answers I got:

pi^e=22.459157718361045464164280552768 e^pi=23.140692632779268996093438846063

e^pi is larger. That's it.

Thomas Yale - 5 years, 6 months ago

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I didn't want to use a calculator/ excel or any other aid.

A Former Brilliant Member - 5 years, 6 months ago
Aritra Das
Mar 5, 2016

e π = π e π ln e = e ln π ln e e = ln π π So we look at the function f ( x ) = ln x x which reaches the maxima at 1 x x 1 ln x x 2 = 0 or, x = e Hence, ln e e ln π π e π π e \begin{aligned} e^\pi &\le=\ge \pi^e \\ \pi \ln e &\le=\ge e \ln \pi \\ \ln e \over e &\le=\ge {\ln \pi \over \pi}\\ \text{So we look at the function } \\ f(x) &= {\ln x \over x}\\ \text{which reaches the maxima at} \\ \frac{{1 \over x} x -1 \ln x}{x^2} &= 0\\ \text{or, } x&=e \\ \text{Hence, }{\ln e \over e} &\ge {\ln \pi \over \pi} \\ e^\pi &\ge \pi^e\\ \end{aligned}

Mardokay Mosazghi
Nov 21, 2015

Use Taylor expaniaon

Chinmay Krishna
Dec 13, 2015

It is true that e^x> x^e if x doesn't equal e

Prajwal Jain
Dec 12, 2015

I did it like, 3^4 is greater than 4^3 As π is greater than e, thus e^π > π^e xD

It doesn't have to be like that all the time. Take 2 and 3 as example. In general, all numbers larger than approximately 2.479 follow the rule: smaller^bigger>bigger^smaller. But that's not the point here, try using derivative and calculus :)

Pavle Medic - 5 years, 6 months ago

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Could you please expand where 2.479 come from? Are you analyzing f(x)=(x+e)^x -x^(x+e)?

Mark Sinsheimer - 5 years, 6 months ago
Yevgeniy Barkalov
Nov 28, 2015

e to the power of any real number other than e itself will be greater than the number to the power of e

Lance Fernando
Nov 24, 2015

Just look at the exponent; pi = 3.14 but Euler's = 2.71. Therefore, 2.71^3.14 > 3.14^2.71

3^2 or 2^3 doesn't apply to just look at the exponents

Gianpiero Delliturri - 5 years, 6 months ago

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What do you mean it doesn't apply? Pi and e are both constants. I solved it using both alternately as base and exponent up to 20 decimal places, just to make absolutely sure.

pi^e=22.459157718361045464164280552768 e^pi=23.140692632779268996093438846063

e^pi's obviously larger.

Thomas Yale - 5 years, 6 months ago

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Lance's solution was to simply look at the exponents and see which is larger. Gianpiero was just providing an example where the rule does not work to show why that isn't a valid way to solve. He wasn't denying that e^pi is the right answer.

Ryan Henderson - 5 years, 6 months ago

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@Ryan Henderson to further the point, actually got the question wrong by rounding to two and three.

Marc Brothers - 5 years, 6 months ago

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