Question

determine a soma dos 18 primeiros termos da PA (1,4,7,...)

Answer 1

P.A(1,4,7
R = 3
a 1 = 1 
A 18 = A 1 + (N - 1 ).3
A 18 = 1 + ( 18 - 1 ).3
A 18 = 1 + 17. 3
A 18 = 1 + 51
A 18 = 52
S 18 = 18.(a 1 + a 18) / 2
S 18 =  18 . ( 1 + 52 ) /2
S 18  = 18 . 53 /2
S 18 = 954/2
S 18 = 477