an是等差数列吧?
S9=a1+a2+.+a5+...+a9
a1=a5-4d,a2=a5-3d,a3=a5-2d.a7=a5+2d,a8=a5+3d,a9=a1+4d.
所以你把所有a1到a9换成a5和d的形式,S9=9a5.
5a3的求法一样.