Find the Next Term 3 , 5 , 9 , 15 , 23

$3$ , $5$ , $9$ , $15$ , $23$

Step 1

Find the first level differences by finding the differences between consecutive terms.

$2, 4, 6, 8$

Step 2

Find the second level difference by finding the differences between the first level differences. Because the second level difference is constant, the sequence is quadratic and given by $a_{n} = a n^{2} + b n + c$ .

$2$

Step 3

Solve for $a$ by setting $2 a$ equal to the constant second level difference $2$ .

Set $2 a$ equal to the constant second level difference $2$ .

$2 a = 2$

Divide each term in $2 a = 2$ by $2$ and simplify.

Divide each term in $2 a = 2$ by $2$ .

$\frac{2 a}{2} = \frac{2}{2}$

Simplify the left side.

Cancel the common factor of $2$ .

Cancel the common factor.

$\frac{2 a}{2} = \frac{2}{2}$

Divide $a$ by $1$ .

$a = \frac{2}{2}$

Simplify the right side.

Divide $2$ by $2$ .

$a = 1$

Step 4

Solve for $b$ by setting $3 a + b$ equal to the first first level difference $2$ .

Set $3 a + b$ equal to the first first level difference $2$ .

$3 a + b = 2$

Substitute $1$ for $a$ .

$3 \cdot 1 + b = 2$

Multiply $3$ by $1$ .

$3 + b = 2$

Move all terms not containing $b$ to the right side of the equation.

Subtract $3$ from both sides of the equation.

$b = 2 - 3$

Subtract $3$ from $2$ .

$b = - 1$

Step 5

Solve for $c$ by setting $a + b + c$ equal to the first term in the sequence $3$ .

Set $a + b + c$ equal to the first term in the sequence $3$ .

$a + b + c = 3$

Substitute $1$ for $a$ and $- 1$ for $b$ .

$1 - 1 + c = 3$

Simplify $1 - 1 + c$ .

Subtract $1$ from $1$ .

$0 + c = 3$

Add $0$ and $c$ .

$c = 3$

Step 6

Substitute the values of $a$ , $b$ , and $c$ into the quadratic sequence formula $a_{n} = a n^{2} + b n + c$ .

$a_{n} = 1 n^{2} - n + 3$

Step 7

Multiply $n^{2}$ by $1$ .

$a_{n} = n^{2} - n + 3$

Step 8

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

$a_{6} = {(6)}^{2} - (6) + 3$

Step 9

Simplify each term.

Raise $6$ to the power of $2$ .

$a_{6} = 36 - (6) + 3$

Multiply $- 1$ by $6$ .

$a_{6} = 36 - 6 + 3$

Step 10

Simplify by adding and subtracting.

Subtract $6$ from $36$ .

$a_{6} = 30 + 3$

Add $30$ and $3$ .

$a_{6} = 33$

Find the Next Term 3 , 5 , 9 , 15 , 23

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