3 -4 * 2 +10 * 3 = 21 = 7 * 3
3的4次方-4 * 3 +10 * 2 = 63 = 7 * 3 2
5日-4 * 3 ^ 4 +10 * 3 = 189 = 7 * 3
...
3 ^（N +2）-4 * 3 ^（N +1）+10 * 3 ^ N = 7 * 3的n次方
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