1 The known series {an} is arithmetic progression. If a 1=7 d=-3, s20 = (a _1+a _ 20) * 20/2 = [7+7+19 * (-3)] * 66.

(22) in arithmetic progression {an}, a4=6, a6= 10.

(1) Find the general term formula of this series.

a6 = a4+2d = 6+2d = 10

d=2

a4 = a _ 1+3d = a _ 1+6 = 6

a_ 1=0

a _ n = a _ 1+(n- 1)d = 2(n- 1)

(2)S _ 30 =(a _ 1+a _ 30)* 30/2 =(0+2 * 29)* 15 = 58 * 15 = 870

(2) Find S30