由1²+2²+3²+。。。+n²=n(n+1)(2n+1)/6 ∵(a+1)³-a³=3a²+3a+1(即(a+1)³=a³+3a²+3a+1) a=1时:2³-1³=3×1²+3×1+1 a=2时:3³-2³=3×2²+3×2+1 a=3时:4³-3³=3×3²+3×3+1 a=4时:5³-4³=3×4²+3×4+1 。。。。。。
a=n时:(n+1)³-n³=3×n²+3×n+1 等式两边相加: (n+1)³-1=3(1²+2²+3²+。。。+n²)+3(1+2+3+。。。+n)+(1+1+1+。。。+1)3(1²+2²+3²+。。。+n²) =(n+1)³-1-3(1+2+3+。。。+n)-(1+1+1+。。。+1)3(1²+2²+3²+。。。+n²) =(n+1)³-1-3(1+n)×n÷2-n6(1²+2²+3²+。。。+n²) =2(n+1)³-3n(1+n)-2(n+1) =(n+1)[2(n+1)²-3n-2] =(n+1)[2(n+1)-1][(n+1)-1] =n(n+1)(2n+1) ∴1²+2²+。。。+n²=n(n+1)(2n+1)/6.