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
