1. Do - Now (-2, 1) & (4, 7) (1, 0) & (0, 4) (-3, -4) & (1, 6)
Find the slope of the line between the given points:
(-2, 1) & (4, 7)
(1, 0) & (0, 4)
(-3, -4) & (1, 6)
m = 1
m = -4

2. 4.5 – Direct Variation x & y vary directly
Two quantities that vary directly have direct variation
Example: x – years & y – age As the years go on…your age goes up
Therefore…as one object does something, so the does the other
Model for direct variation: y = kx
k (is a #) called the constant of variation

3. What is the constant of variation?
y = 2x
y = –4x
y = x
y = x
k = 2
The slope is also a constant of variation.
k = –4
The graph of y = kx is a line through the origin. x y

4. Writing a direct variation equation:
The variables x & y vary directly, write an equation when x = 5 and y = 20.
y = kx
What if x = 4 and y = 12?
20 = k(5)
y = kx
4 = k
12 = k(4)
The equation is… y = 4x
3 = k
The equation is… y = 3x
Find the value of y when x = 10.
y = 3(10) = 30