Question

What is the sum of a geometric sequence 1, 3, 9, ... if there are 12 terms?

Answer 1

We know that
a1=1
a2=3
a3=9
a2/a1=3/1----> 3
a3/a2=9/3----> 3
common ration r is equal to 3
number of terms n  is 12

The Sum of geometric series is given by the formula
Sum=a1*[1-r^n]/[1-r]
Sum=1*[1-3^12]/[1-3]-----> Sum=[1-3^12]/[1-3]----> [3^12-1]/[3-1]
Sum=531440/2-----> 265720

the answer is
265720