geometric sequences and series:

Question

1. Find the third term in a geometric sequence if a=8 and r=-1. Use the formula an=ar^n-1.
2. Find the sum of the first 3 terms of a geometric sequence if a=5 and r=3. Use the formula

Sn = a(1-r^n)/1-r

Answer 1

The formulas are given, all you need to do is plug in the numbers and evaluate.

a₃ = 8(-1)³

S₃ = 5(1-3³)/(1-3)