Find the Equation Using Point-Slope Formula (1,15) , (3,24)

$(1, 15)$ , $(3, 24)$

Step 1

Find the slope of the line between $(1, 15)$ and $(3, 24)$ using $m = \frac{y_{2} - y_{1}}{x_{2} - x_{1}}$ , which is the change of $y$ over the change of $x$ .

Slope is equal to the change in $y$ over the change in $x$ , or rise over run.

$m = \frac{change in y}{change in x}$

The change in $x$ is equal to the difference in x-coordinates (also called run), and the change in $y$ is equal to the difference in y-coordinates (also called rise).

$m = \frac{y_{2} - y_{1}}{x_{2} - x_{1}}$

Substitute in the values of $x$ and $y$ into the equation to find the slope.

$m = \frac{24 - (15)}{3 - (1)}$

Simplify.

Simplify the numerator.

Multiply $- 1$ by $15$ .

$m = \frac{24 - 15}{3 - (1)}$

Subtract $15$ from $24$ .

$m = \frac{9}{3 - (1)}$

Simplify the denominator.

Multiply $- 1$ by $1$ .

$m = \frac{9}{3 - 1}$

Subtract $1$ from $3$ .

$m = \frac{9}{2}$

Step 2

Use the slope $\frac{9}{2}$ and a given point $(1, 15)$ to substitute for $x_{1}$ and $y_{1}$ in the point-slope form $y - y_{1} = m (x - x_{1})$ , which is derived from the slope equation $m = \frac{y_{2} - y_{1}}{x_{2} - x_{1}}$ .

$y - (15) = \frac{9}{2} \cdot (x - (1))$

Step 3

Simplify the equation and keep it in point-slope form.

$y - 15 = \frac{9}{2} \cdot (x - 1)$

Step 4

Solve for $y$ .

Simplify $\frac{9}{2} \cdot (x - 1)$ .

Rewrite.

$y - 15 = 0 + 0 + \frac{9}{2} \cdot (x - 1)$

Simplify by adding zeros.

$y - 15 = \frac{9}{2} \cdot (x - 1)$

Apply the distributive property.

$y - 15 = \frac{9}{2} x + \frac{9}{2} \cdot - 1$

Combine $\frac{9}{2}$ and $x$ .

$y - 15 = \frac{9 x}{2} + \frac{9}{2} \cdot - 1$

Multiply $\frac{9}{2} \cdot - 1$ .

Combine $\frac{9}{2}$ and $- 1$ .

$y - 15 = \frac{9 x}{2} + \frac{9 \cdot - 1}{2}$

Multiply $9$ by $- 1$ .

$y - 15 = \frac{9 x}{2} + \frac{- 9}{2}$

Move the negative in front of the fraction.

$y - 15 = \frac{9 x}{2} - \frac{9}{2}$

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

Add $15$ to both sides of the equation.

$y = \frac{9 x}{2} - \frac{9}{2} + 15$

To write $15$ as a fraction with a common denominator, multiply by $\frac{2}{2}$ .

$y = \frac{9 x}{2} - \frac{9}{2} + 15 \cdot \frac{2}{2}$

Combine $15$ and $\frac{2}{2}$ .

$y = \frac{9 x}{2} - \frac{9}{2} + \frac{15 \cdot 2}{2}$

Combine the numerators over the common denominator.

$y = \frac{9 x}{2} + \frac{- 9 + 15 \cdot 2}{2}$

Simplify the numerator.

Multiply $15$ by $2$ .

$y = \frac{9 x}{2} + \frac{- 9 + 30}{2}$

Add $- 9$ and $30$ .

$y = \frac{9 x}{2} + \frac{21}{2}$

Step 5

Reorder terms.

$y = \frac{9}{2} x + \frac{21}{2}$

Step 6

List the equation in different forms.

Slope-intercept form:

$y = \frac{9}{2} x + \frac{21}{2}$

Point-slope form:

$y - 15 = \frac{9}{2} \cdot (x - 1)$

Step 7

Find the Equation Using Point-Slope Formula (1,15) , (3,24)

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