Question

Determine a soma dos termos de cada PA b) (86,83,80,...35

Answer 1

a1 = 86
a2 = 83
r = 83 - 86 = -3 ***
an = 35
35 = a1 + (n-1)r
35 = 86 + (n-1)(-3)
35 = 86 - 3n + 3
35 = 89 - 3n
35-89 = -3n
-54 = -3n
3n = 54
n = 54/3 = 18 ****
S18 = ( 86 + 35)* 9
S18 = 121* 9 = 1089 ***