What is 1-2+4-8+-a10 in evaluate series

This is a geometric sequence, the sum of which can always be expressed as:

s (n) = a (1-r^n) / (1-r), a=initial term, r=common ratio, n=term number ...

The common ratio is defined as the constant, r, which is the ratio of any term in relation to the previous term ... in this case:

-2/1=4/-2=-8/4=r=-2 and a is obviously 1 so:

s (n) = (1 - (-2) ^n) / (1--2)

s (n) = (1 - (-2) ^n) / 3 so the sum of the first 10 terms is:

s (10) (1 - (-2) ^10) / 3

s (10) = - 341