Question

a1=-1 y a5=7 , calcular la suma de los 15 primeros terminos

Answer 1

A5 = A1 + r(n-1)
7 = -1 + r (4)
8 = 4r
r = 2

A15 = A1 + r (n-1)
A15 = -1 + 2 (14)
A15 = 27

S = $\frac{A1 + An}{2} . n$
S = $\frac{-1 + 27}{2} . 15$
S = $\frac{26}{2} . 15$
S = 13 . 15 = 195