Question

Calcule a soma dos seis primeiros termos dá P.G. ( -5,10,-20,...)

Answer 1

sn=a1.(q^n-1)/q-1      use a formula   q=10/-5    q=-2
Sn=-5(-2^6-1)/-2-1
Sn=-5(64-1)/-3
Sn=-5.63/-3
sn=-315/-3
sn=105