Question

Which equation should be used to calculate S9 for the arithmetic sequence an=3n-1

Answer 1

Answer:

Option A is correct.

The equation = $S_9 =\frac{9}{2} (2+26)$.

Explanation:

Sum of the terms of an arithmetic series is given by

$S_n = \frac{n(a_1+a_n)}{2}$,                  ......[1]

where n is the number of terms, $a_1$ is the first term and  $a_n$is the last term.

Given the formula for nth term in the arithmetic sequence is, $a_n = 3n-1$

for n =1

$a_1 = 3 \cdot 1 -1 = 3-1 =2$

for n =9

$a_9= 3 \cdot 9 -1 = 27-1 =26$

Then, put these values in equation [1] for n =9;

$S_9 = \frac{9 \cdot (a_1+a_9)}{2} = \frac{9(2+26)}{2}$ or

$S_9 =\frac{9}{2} (2+26)$

Therefore, the equation is; $S_9 =\frac{9}{2} (2+26)$

Answer 2

Answer:

1. D

2. A

3. C

4. D

5. A

Step-by-step explanation: