辛夷坞

涧户寂无人,纷纷开且落

By - Christen

等差数列求和公式推导

倒序相加
Sn=1+2+3+……+(n-1)+n
Sn=n+(n-1)+(n-2)+……+2+1
2Sn=(1+n)+(2+n-1)+(3+n-2)+……+(n-1+2)+(n+1)=(n+1)+(n+1)+(n+1)+……+(n+1)+(n+1)
一共n项(n+1):2Sn=n(n+1)
Sn=n(n+1)/2

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