math please help me

Question

The following problem refers to an arithmetic sequence. If  and , find  and .

Answer 1

a1 = 1,, d=-3, find a19 and S19

a1 = 1

a2 = 1-3 =-2

a3 = a2+d = -2-3 = -5

a4 = a3 +d = -5-3= -8

a5 = a4 +d =-8-3 = -11

an= a1 -3(n-1)

a29 = a1- 3(28) = 1-84 =-83

a29 =-83

a19 = a1 -3(18) = 1-54 =-53

s19 =?

sn = n(a1 + an)/2 = 19(1+a19)/2 = 19(1-53)/2 = -19x52/2 =-26x19=-494