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 explain.

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

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