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