Question

Find the sum of 10 terms if the nth term of a sequence is $3^n$- 2.

• A. ( - 1) - 20
• B. ( - 1) - 20
• C. ( - 1) - 20
• D. ( - 1) - 30

Answer 1

The correct option is C

( - 1) - 20

$S_n$ = ∑$t_n$

$S_{10}$ =${\sum }_{n=1}^{10}$($3^n$ - 2)

= ${\sum }_{n=1}^{10}$$3^n$ - ${\sum }_{n=1}^{10}$2

= $\frac{3(3^{10}-1)}{2}$ - $2\times 10$

= $\frac{3}{2}$ ($3^{10}$ - 1) - 20