Question

Calcule a soma dos 10 primeiros termos dessa PG. ( -1, 2 ,-4, ...)

Answer 1

$P.G~(-1,2,-4,...)$

$a_{1}=-1\\a_{2}=2\\\\q=a_{2}/a_{1}=2/(-1)=-2$

$S_{n}=a_{1}(q^{n}-1)/(q-1)\\S_{10}=a_{1}(q^{10}-1)/(q-1)\\S_{10}=(-1)*([-2]^{10}-1)(-2-1)\\S_{10}=(-1)*(1024-1)/(-3)\\S_{10}=-1023/(-3)\\S_{10}=341$