1. 2.4 Writing the Equation of a Line

2. Review of Slope-Intercept Form
The slope-intercept form of a linear equation is
y = mx + b.
m represents the slope
b represents the y-intercept

3. Review of Slope-Intercept Form
Name the slope and y-intercept of each equation:
m = -4, b =3
m = , b = 5
Rewrite as y = -8x , m = -8, b =
Rewrite as y = 2x - 5, m = 2, b = -5
Rewrite as y = x + 0, m = , b = 0
Rewrite as y = 0x + 5, m = 0, b = 5
y = -4x + 3
y = 5 – x
8x + y =
4x - 2y =10
y = x
y = 5

4. Write the equation of a line given...
1. Slope and y-intercept
2. Graph
3. Slope and one point
4. Two points
5. x - and y-intercepts

5. Find the Equation of the Line Given the Slope and y-intercept
Substitute m and b into y = mx + b
m = -3, b = 1
m = -2, b = -4
m = 0, b = 10
m = 1, b = 0
m = 0, b = 0
y = -3x + 1
y = -2x - 4
y = 0x + 10, y = 10
y = 1x + 0, y = x
y = 0x + 0, y = 0

6. Find the Equation of a Line Given the Graph
Find the y-intercept from the graph.
Count the slope from the graph.
To write the equation of the line, substitute the slope and y-intercept in the slope-intercept form of the equation.

7. Example 1
b = -3
m =
y = x - 3 x y +2 +3

8. Example 2
b = 1
m =
y = x + 1 x y +1 -2

9. Example 3
b = 4
m = 0/1= 0
y = 0x + 4, y = 4 x y

10. Find the Equation of a Line Given the Point and the Slope
Use the Point-Slope Formula:
is the given point
Substitute m and into the formula

11. Example
Write the equation of the line with slope = -2 and passing through the point (3, -5).
Substitute m and into the Point-Slope Formula.

12. Find the Equation of the Line Given Two Points
Calculate the slope of the two points.
Use one of the points and the slope to substitute into the Point-Slope formula.

13. Example
Write the equation of the line that goes through the points (3, 2) and (5, 4).

14. Find the Equation of the Line Given the x- and y - intercepts
Write the intercepts as ordered pairs.
The x-intercept 4 is the ordered pair (4, 0).
The y-intercept -2 is the ordered pair (0, -2).
Calculate the slope.
Substitute the slope and the y-intercept (b) into the slope-intercept formula.

15. Example Write the equation of the line with x-intercept 3
and y-intercept 2.
x-intercept 3 = (3, 0); y-intercept 2 = (0, 2)
Slope:
y = mx + b