（k+1）²=k²+2k+1,改变公式的写法,得（k+1）²- k²=2k+1.对上面的等式,依次令k=1,2,3,……,n,得2²-1²=2×1+1,3²-2²=2×2+1,4²-3²=2×3+1,……记S1=1+2+3+··+n,S1就等于n（n+1）除
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