Which is bigger, $3^{21}$ or $21^{3}$?

How to work it out

1. 1

   Compute both exactly: $3^{21}$ = 10460353203 and $21^{3}$ = 9261.

2. 2

   So $3^{21}$ is bigger.

3. 3

   The reason: taking logs, $a^{b}$ against $b^{a}$ is the same as $\frac{\ln a}{a}$ against $\frac{\ln b}{b}$. The function $\frac{\ln x}{x}$ rises until $x = e \approx 2.718$ and falls afterwards. Both 3 and 21 are past e, where the function is decreasing, so the larger exponent loses and $3^{21}$ > $21^{3}$.

   $$\frac{\ln 3}{3}\ >\ \frac{\ln 21}{21}$$

Questions

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