s7=(a1+a7)*7/2=3(a3+a7)所以7(a1+a1+6d)=6(a1+2d+a1+6d)所以a1=3ds9/a4=[(a1+a9)*9/2]/a4=(a1+a1+8d)*9/[2(a1+3d)]=14d*9/12d=21/2
