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Which is bigger, 5185^{18} or 18518^{5}?

5185^{18} is bigger: 5185^{18} = 3.81469 × 101210^{12} and 18518^{5} = 1889568.

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The curve ln x / x, which decides every comparison of a^b with b^a2468101214161820-0.10.10.20.30.4x = 5x = 18peak at eln x / x
The curve ln x / x, which decides every comparison of a^b with b^a. 5^18 against 18^5 is the same question as which of the two marked points is higher; the curve peaks at x = e.

How to work it out

  1. 1

    Compute both exactly: 5185^{18} = 3814697265625 and 18518^{5} = 1889568.

  2. 2

    So 5185^{18} is bigger.

  3. 3

    The reason: taking logs, aba^{b} against bab^{a} is the same as ln⁡aa\frac{\ln a}{a} against ln⁡bb\frac{\ln b}{b}. The function ln⁡xx\frac{\ln x}{x} rises until x=e≈2.718x = e \approx 2.718 and falls afterwards. Both 5 and 18 are past e, where the function is decreasing, so the larger exponent loses and 5185^{18} > 18518^{5}.

    ln⁡55 > ln⁡1818\frac{\ln 5}{5}\ >\ \frac{\ln 18}{18}
LaTeX source of the formulas on this page
\[ 5^{18} = 3814697265625\ >\ 18^{5} = 1889568 \]
\[ \frac{\ln 5}{5}\ >\ \frac{\ln 18}{18} \]

Questions

Which is bigger, 5185^{18} or 18518^{5}?

5185^{18} is bigger: 5185^{18} = 3.81469 × 101210^{12} and 18518^{5} = 1889568.

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