SOLUTION: Sn=a(subscript1)x(1-r^n/1-r) is the formula if given are positive= 1/9+8/9+etc. But what if the given are negative= -1-8-64-512...-(8)^n-1 or subtraction. Is the formula the same?

Algebra ->  Sequences-and-series -> SOLUTION: Sn=a(subscript1)x(1-r^n/1-r) is the formula if given are positive= 1/9+8/9+etc. But what if the given are negative= -1-8-64-512...-(8)^n-1 or subtraction. Is the formula the same?      Log On


   

Question 1132939: Sn=a(subscript1)x(1-r^n/1-r) is the formula if given are positive= 1/9+8/9+etc.
But what if the given are negative= -1-8-64-512...-(8)^n-1 or subtraction. Is the formula the same? or do is it flip and turn int0 a1x(1+r^n/1+r)?
Answer by MathLover1(20850) About Me  (Show Source):
You can put this solution on YOUR website!

if r%3C1 you would use S%5Bn%5D=a%5B1%5D%28%281-r%5En%29%2F%281-r%29%29
if r%3E1 you would use S%5Bn%5D=a%5B1%5D%28%28r%5En-1%29%2F%28r-1%29%29