very难!
问题描述：
very难!
1的2次方+2的2次方+3的2次方+4的2次方+5的2次方+6的2次方+.(n-3)的2次方+(n-2)的2次方+(n-1)的2次方+n的2次方!
答
n(n+1)(2n+1)/6
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