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Which is bigger, 8158^{15} or 15815^{8}?

8158^{15} is bigger: 8158^{15} = 3.51843 × 101310^{13} and 15815^{8} = 2562890625.

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The curve ln x / x, which decides every comparison of a^b with b^a24681012141618-0.3-0.2-0.10.10.20.30.4x = 8x = 15peak at eln x / x
The curve ln x / x, which decides every comparison of a^b with b^a. 8^15 against 15^8 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: 8158^{15} = 35184372088832 and 15815^{8} = 2562890625.

  2. 2

    So 8158^{15} 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 8 and 15 are past e, where the function is decreasing, so the larger exponent loses and 8158^{15} > 15815^{8}.

    ln⁡88 > ln⁡1515\frac{\ln 8}{8}\ >\ \frac{\ln 15}{15}
LaTeX source of the formulas on this page
\[ 8^{15} = 35184372088832\ >\ 15^{8} = 2562890625 \]
\[ \frac{\ln 8}{8}\ >\ \frac{\ln 15}{15} \]

Questions

Which is bigger, 8158^{15} or 15815^{8}?

8158^{15} is bigger: 8158^{15} = 3.51843 × 101310^{13} and 15815^{8} = 2562890625.

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