Question

The fifth term of a geometric series is 405 and the sixth term is 1215. Find the sum of the first nine terms. Don't know how to find the first term please explain.

Answer 1

Since the sixth term is 1215 and the 5th term is 405, we know each term is 3 times more than the last.
Let 'n' represent the first term, then the sum of the first 9 terms can be written as:
n+3n+9n+27n+81n+243n+729n+2187n+6561n
which sinplifies to 9841n
since each term is 3 times the last, the first term must be 405 divided by 3, 4 times in total, which is 5
Now subsitute 'n' for 5, and we get 9841(5)
which is 49205 which is the answer.

Hope this helps