Mathematics-基础:1+2+3+……+n

设Sn=1+2+3+……+n-1+n
则有Sn=n+n-1+……+3+2+1
两式相加得2Sn=(n+1)+(n+1)+……+(n+1)
2Sn=n×(n+1)
Sn=n×(n+1)/2