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 = 95/5 = 19 ****
a19 = 15 + 18r
a19 = 15 + 18 ( 5)
a19 = 15 + 90
a19 = 105
S19 = ( 15 + 105)*19/2
S19 =120 * 19/2 = 2280/2 = 1140 ****