Question

Need help. Don't have a calculator.

Find the sum of the first n terms
1+-2+4+-8+16+-32+... (10 terms)

Formula: a-a*r^n / 1-r

Answer 1

the formula is

`(a_1(1-r^n))/(1-r)`
a1=first term
n=which term
r=common ratio
we see the common ratio is -2
frs term is 1
and we go to 10th term so n=10


`(1(1-(-2)^(10)))/(1-(-2))`

`(1(1-1024))/(1+2)`

`(1(-1023))/(3)`
-341
the sum is -341