Question

Quantos termos tem a PA (10, 15, 20, ..., 885) ?

Answer 1

a1 = 10
an = 885
r = 5
n = ?

an = a1 + (n-1).r
885 = 10 + (n-1).5
885 = 10 + 5n - 5
885 = 5 + 5n
885 - 5 = 5n
880 = 5n
n = 880/5
n = 176