An=a1+d(n-1)
a5=a1+d(5-1 a2=a1+d(2-1) a7=a1+d(7-1) a3=a1+d(3-1)
a5=a1+4d a2=a1+d a7=a1+6d a3=a1+2d
a1+4d+2(a1+d)=-36 a1+4d+2a1+2d=-36 3a1+6d=-36
a1+6d-a1+2d=-24 8d=-24 d=-3
3a1+6(-3)=-36 a9=a1+d(9-1) Sn=n(2a1+d(n-1))/2
3a1-18=-36 a9=-6-24 S8=8(-12-21)/2
3a1=-36+18 a9=-30 S8=4(-33)
3a1=-18 S8=-132
a1=-6