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Which is bigger, 797^{9} or 979^{7}?

797^{9} is bigger: 797^{9} = 40353607 and 979^{7} = 4782969.

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The curve ln x / x, which decides every comparison of a^b with b^a24681012-0.8-0.6-0.4-0.20.20.4x = 7x = 9peak at eln x / x
The curve ln x / x, which decides every comparison of a^b with b^a. 7^9 against 9^7 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: 797^{9} = 40353607 and 979^{7} = 4782969.

  2. 2

    So 797^{9} 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 7 and 9 are past e, where the function is decreasing, so the larger exponent loses and 797^{9} > 979^{7}.

    ln⁡77 > ln⁡99\frac{\ln 7}{7}\ >\ \frac{\ln 9}{9}
LaTeX source of the formulas on this page
\[ 7^{9} = 40353607\ >\ 9^{7} = 4782969 \]
\[ \frac{\ln 7}{7}\ >\ \frac{\ln 9}{9} \]

Questions

Which is bigger, 797^{9} or 979^{7}?

797^{9} is bigger: 797^{9} = 40353607 and 979^{7} = 4782969.

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