Question

What is the sum of the geometric sequence 3,15,75... if there are 7 terms ?

Answer 1

Answer:

58593

Step-by-step explanation:

The sum to n terms of a geometric sequence is

$S_{n}$ = $\frac{a(r^n-1)}{r-1}$

where a is the first term and r the common ratio

r = 15 ÷ 3 = 75 ÷ 15 = 5 and a = 3, hence

$S_{7}$ = $\frac{3(5^7-1)}{5-1}$

= $\frac{3(78125-1)}{4}$ = $\frac{3(78124)}{4}$ = 58593