Sm=n,Sn=m,Sm+n=?

只就m不等于时计算
Sm=ma1+(1/2)m(m-1)d=n (1) Sn=na1+(1/2)n(n-1)d (2)
n*(1)-M*(2)：(1/2)mn(m-n)d=n^2-m^2=(n-m)(n+m) mnd=-2m-2n
Sm+n=a1+a2+…+am+a(m+1)+a(m+2)+…+a(m+n)
=n+a1+md+a1+(m+1)d+…+a1+(m+n-1)d
=n+mnd+a1+a1+d+a1+2d+…+a1+(n-1)d
=n+mnd+a1+a2+…+an
=m+n+mnd
=-m-n