Precalculus Examples

$- 1$ , $2$ , $5$ , $8$ , $11$ , $14$

Step 1

This is the formula to find the sum of the first $n$ terms of the sequence. To evaluate it, the values of the first and $n$ th terms must be found.

$S_{n} = \frac{n}{2} \cdot (a_{1} + a_{n})$

Step 2

This is an arithmetic sequence since there is a common difference between each term. In this case, adding $3$ to the previous term in the sequence gives the next term. In other words, $a_{n} = a_{1} + d (n - 1)$ .

Arithmetic Sequence: $d = 3$

Step 3

This is the formula of an arithmetic sequence.

$a_{n} = a_{1} + d (n - 1)$

Step 4

Substitute in the values of $a_{1} = - 1$ and $d = 3$ .

$a_{n} = - 1 + 3 (n - 1)$

Step 5

Simplify each term.

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Step 5.1

Apply the distributive property.

$a_{n} = - 1 + 3 n + 3 \cdot - 1$

Step 5.2

Multiply $3$ by $- 1$ .

$a_{n} = - 1 + 3 n - 3$

Step 6

Subtract $3$ from $- 1$ .

$a_{n} = 3 n - 4$

Step 7

Substitute in the value of $n$ to find the $n$ th term.

$a_{6} = 3 (6) - 4$

Step 8

Multiply $3$ by $6$ .

$a_{6} = 18 - 4$

Step 9

Subtract $4$ from $18$ .

$a_{6} = 14$

Step 10

Replace the variables with the known values to find $S_{6}$ .

$S_{6} = \frac{6}{2} \cdot (- 1 + 14)$

Step 11

Divide $6$ by $2$ .

$S_{6} = 3 \cdot (- 1 + 14)$

Step 12

Add $- 1$ and $14$ .

$S_{6} = 3 \cdot 13$

Step 13

Multiply $3$ by $13$ .

$S_{6} = 39$

Step 14

Convert the fraction to a decimal.

$S_{6} = 39$

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