Answer:
20,176,803
Step-by-step explanation:
This geometric series has a first term of 3 and a common ratio of 21/3 = 7. The formula for the sum of n terms of a series with first term a1 and common ratio r is ...
Sn = a1·(r^n -1)/(r -1)
For a1=3, n=9, and r=7, the sum is ...
S9 = 3·(7^9 -1)/(7 -1) = 3·40353606/6 = 20,176,803
The sum of the first 9 terms is 20,176,803.
_____
The first 9 terms are ...
3 +21 +147 +1029 +7203 +
50,421 +352,947 +2,470,629 +17,294,403
