Question

Determine a soma dos 20 primeiros termos da PA (-4/5, -3/5, ...)

Answer 1

Resposta:

S_20=22

Explicação passo-a-passo:

$R=(-\frac{3}{5})-(-\frac{4}{5}=\frac{-3+4}{5}=\frac{1}{5}\\a_1=-\frac{4}{5}\\a_{20}=-\frac{4}{5}+19*(\frac{1}{5})\\a_{20}=-\frac{4}{5}+\frac{19}{5}\\a_{20}=\frac{-4+19}{5}=\frac{15}{5}=3\\S_{20}=\frac{(-\frac{4}{5}+3)*20}{2}\\S_{20}=\frac{-\frac{4*20}{5}+60}{2}\\S_{20}=\frac{-4*4+60}{2}\\S_{20}=\frac{-16+60}{2}\\S_{20}=\frac{44}{2}\\\boxed{S_{20}=22}$