Question

What is the sum of the geometric sequence 4, 16, 64, … if there are 8 terms?

Answer 1

for a geometric series:\[a, ar, ar^2, ..., ar^n\]the sum is defined as:\[\sum_{k=0}^nar^k=\frac{a(1-r^{n+1})}{1-r}\]in your case a (the first term of the series) is 4, and r (the multiplying factor) is also 4. n is the number of terms which is 8.