Question

Find the sum of first 8 terms of the G.P. 1, 3, 9, 27, 81, .........

Answer 1

Here, first term $$a = 1$$, common ratio $$r = 3$$ and $$n = 8$$.

Since $$r > 1$$, use $$S_n = \dfrac{a(r^n - 1)}{r - 1}$$.

$$S_8 = \frac{1 \times (3^8 - 1)}{(3 - 1)}$$

$$= \frac{(6561 - 1)}{2}$$

$$= \frac{6560}{2}$$

$$= 3280$$

Hence, the required sum is 3280.