1. 2.4A Writing Equations of Lines
Algebra II

2. We’ve learned to graph given an equation.
Now we’ll learn to write the equation given the graph
There are three ways.
It all depends on what information you are given as to which process you use.

3. A.)Given the slope, m, and the y-intercept, b, use the equation y=mx+b
Ex. 1
The y-intercept is -3
b=-3
The slope is 4/3
The equation is:
y = 4/3x – 3 3 4

4. B.) If you are given slope, m, and a point (x1,y1) on the line Use Point Slope Form:
y – y1 = m ( x – x1)

5. Ex. 2 Write an equation of a line containing the point (1,2) with slope of -1/2.
Use (x1,y1) = (1,2) & m = -1/2
y – 2 = -1/2 ( x – 1)
Now you can simplify to the slope intercept form
y – 2 = -1/2 x + ½
y = -1/2 x + 5/2

6. C.) Given two points Use slope formula first.
Then, either use point-slope formula or slope-intercept form twice.

7. Ex. 3 Given two points (-2,2) & (3,7)
Find the slope:
m=1
Plug this slope and one of the points into the point slope formula.
y – 2 = 1 ( x – (-2))
y – 2 = x + 2
y = x (put the equation into slope intercept form)
(3,7)
(-2,2)

9. Assignment