Answer
Sum of first ten terms $S_{10}$ = 6820
Work Step by Step
The given sequence = -20, 40, -60, 80,....................
This is a geometric sequence so the sum find by formula $S_{n}$ = $a_{1}$$\frac{1 - r^{n}}{1 - r}$
In the given sequence first term $a_{1}$ = -20
Common ratio (r) = -2
Sum of first ten terms $S_{10}$ =-20$\times$$\frac{1 - (-2)^{10}}{1 - (-2)}$
= -20$\times$$\frac{-1023}{3}$
= -20$\times$ -341
= 6820
