Question

soma do oitavo termo da pg (-5, 10, -20).

Answer 1

$P.G(-5,10,-20...) \\ \\ \ S_{n}= S_{8}=? \\ a_{1}=-5 \\ n=8 \\ q=-2 \\ \\ \\ S_{n} = \frac{ a_{1} ( q^{n} -1)}{q-1} \\ \\ \\ S_{n}= \frac{-5( (-2)^{8}-1) }{-2-1} \\ \\ S_{n}= \frac{-5(256-1)}{-3} \\ \\ S_{n}= \frac{-5+255}{-3} \\ \\ S_{n}= \frac{255}{-3} \\ \\ \\ S_{n}= -85$