The first term of a geometric series is 4 and the sum of the first three terms of the series is 64. Find the sum of the first eight terms of the series.

Answer 1

Start with the Sum of n terms first.

sum= firstterm* ((1-r^n)/(1-r))

so first term=4, and n=3, solve for r.

Now that r is known, use the formula again to solve for sum of eight terms.