Question

Calcule a soma dos 20 primeiros termos de cada P.A

B) (-1/2),-1/4,0)

Answer 1

a1 = -1/2
a2 = -1/4
a3 = 0 
r = -1/4 - ( - 1/2)  = - 1/4 + 1/2 = ( -1 + 2)/4  = 1/4 ***
a1 + 19r =  -1/2 + 19 ( 1/4) = - 1/2 + 19/4 = ( -2 + 19)/4 =17/4 ***
n = 20
S20 =[ ( -1/2 ) + ( 17/4)]* 10
S20 =[ ( -2 + 17) /4]* 10 =  15/4 * 10/1 = 150/4 = 75/2 ***