Question

calcule a soma dos termos da p.a. (15,20,25,...105)

Answer 1

a1 = 15
a2 = 20
r = 20 - 15 = 5 ***
an = 105
105 = 15 + ( n - 1)5
105 = 15 + 5n - 5
105 - 15 + 5 = 5n
5n = 95
n = 19 ***
Sn = ( 15 + 105)*n/2
Sn =  120 * n/2 = 120 * 19/2 = 120 * 9,5 = 2340