$3^{500} = 243^{100}$
$243 \equiv 12 $ (mod21)
$243^{100} \equiv 12^{100} $ (mod21)
จาก $12^{100} = 144^{50}$
$144 \equiv -3$ (mod21)
$144^{50} \equiv 3^{50}$ (mod21)
$ \equiv 243^{10}$ (mod21)
$ \equiv 12^{10}$ (mod21)
$\equiv (-3)^5$ (mod21)
$\equiv -243$ (mod21)
$\equiv -12$ (mod21)
$\equiv 9$ (mod21)
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