Помогите с прогрессией . аn =3n-4 . S20 -?

Question

Помогите с прогрессией .
аn =3n-4 . S20 -?

Answer 1

A1 = - 1
a2 = 2
d = a2 - a1 = 3
a20 = a1 + 19d = - 1 + 19*3 = 56
S20 - ?

S20 = 10*(a1 + a20) = 10*(56 - 1) = 55*10 = 550