What is the sum of the geometric sequence 8, -16, 32 . if there are 15 terms?

Question

anonymous · Answer

ooh careful on this one now it is  geometric not aritmatic and it is a sum not a sequence :P

anonymous · Answer

yesss ic this lol but these are the ones that im just like....... kill me now lol

anonymous · Answer

ar0+ar1+ar2

so   a is 8 
now we try to find r by looking at its 2nd term
-16=8*r  sfr      r=-16/8  which is -2 right? with me so far?

anonymous · Answer

yup :P

anonymous · Answer

to find sum  we use 

a*r^(n+1)/(r-1)-a/(r-1)   that is just a formula     n is how many terms we want to add up but we start at 0 you want 15 terms so we subtract 1  so  
n=15   a=8   and r=-2 :) just plug in the numbers

anonymous · Answer

87384