Question

Calcule a soma dos 5 primeiro termos da p.g. (7,21,....)?

Answer 1

$p.g. (7,21,....)\\\\ S_n=?\\ a_1=7\\ q= \frac{21}{7} =3\\ n=5\\\\ S_n= \frac{a_1*(q^{n}-1)}{q-1} \\\\ S_5= \frac{7*(3^{5}-1)}{3-1} \to \\\\ S_5= \frac{7*(243-1)}{2} \to\\\\ S_5= \frac{7*242}{2} \to\\\\ S_5= \frac{1694}{2} \to \\\\ S_n=847$