Question

помогите сроочно надо​

Answer 1

a(n) = a1 +d(n-1)

a(2) = a1 + d(2-1) = a1 + d

15.8 = 9 + d

d = 15.8 - 9 = 6.8

a3 = 9 + 6.8(3-1) = 9 + 6.8*2 = 22.6

a4 = 9 + 6.8(4-1) = 9 + 6.8*3 = 29.4

S4 = (n(a1+a(n))/2 = (4(9+29.4))/2 = 76.8