Question

Determine a soma dos 18 primeiros termos da PA (6,10,14,...)

Answer 1

Sn = n . ( a1 + an ) / 2
________________///

a18 = 6 + ( 18 - 1 ) . 4
a18 = 6 + 17 . 4
a18 = 6 + 68
a18 = 74
________________/////

Sn = n . ( a1 + an ) / 2
S18 = 18 . ( 6 + 74 ) / 2
S18 = 18 . 80 / 2
S18 = 1440 / 2
S18 = 720

R:>: 720 a soma dos 18 termos .