geometric progression

Question

The first term is 5, the common ratio is 3 and the last term is 98415. Use the formula a_n = a₁ x r^n-1 to find how many terms there are in the progression

Answer 1

98415=5*3^n-1

19683=3^n-1

19683=3*3*3*3*3*3*3*3*3=3⁹

3⁹=3^n-1

9=n-1

n=9+1

n=10 terms

Answer 2

A_n = A₁ x 3^n-1

 

98415 = 5 x 3^n-1

 

19683 = 3^n-1

 

n = 9