(Sorry, but I haven't mastered using the sigma notation in these forums yet).
Find the sum of the following infinite series: (n=0)^(inf) SIGMA ((pi)cos(n))/(5^n).
I tried using the formula S=(a1)/(1-r).
I know that a=pi, but I can't find "r." The equation doesn't have a consistent rate. Should I be using a different method?