1. The variables x and y vary directly if, for a constant k,
USING DIRECT AND INVERSE VARIATION
DIRECT VARIATION
The variables x and y vary directly if, for a constant k, y x = k, or
y = kx,
k  0.

2. The variables x and y vary inversely, if for a constant k,
USING DIRECT AND INVERSE VARIATION
INDIRECT VARIATION
The variables x and y vary inversely, if for a constant k, k x = y, or
xy = k,
k  0.

3. MODELS FOR DIRECT AND INVERSE VARIATION
USING DIRECT AND INVERSE VARIATION
MODELS FOR DIRECT AND INVERSE VARIATION
DIRECT VARIATION
INVERSE VARIATION k x = y
y = kx
k > 0
k > 0

4. When x is 2, y is 4. Find an equation that relates x and y
Using Direct and Inverse Variation
When x is 2, y is 4. Find an equation that relates x and y
in each case.
x and y vary directly
SOLUTION y x = k
Write direct variation model. 4 2 = k
Substitute 2 for x and 4 for y.
2 = k
Simplify.
An equation that relates x and y is = 2, or y = 2x. y x

5. When x is 2, y is 4. Find an equation that relates x and y
Using Direct and Inverse Variation
When x is 2, y is 4. Find an equation that relates x and y
in each case.
x and y vary inversely
SOLUTION
xy = k
Write inverse variation model.
(2)(4) = k
Substitute 2 for x and 4 for y.
8 = k
Simplify.
An equation that relates x and y is xy = 8, or y = 8 x

6. Compare the direct variation model and the inverse
Comparing Direct and Inverse Variation
Compare the direct variation model and the inverse
variation model you just found using x = 1, 2, 3, and 4.
Make a table using y = 2x and y = 8 x
SOLUTION 8 x
y =
y = 2x 1 2 3 4 6
Direct Variation: k > 0.
As x increases by 1,
y increases by 2.
Inverse Variation: k > 0.
As x doubles (from 1 to 2),
y is halved (from 8 to 4).

7. Compare the direct variation model and the inverse
Comparing Direct and Inverse Variation
Compare the direct variation model and the inverse
variation model you just found using x = 1, 2, 3, and 4.
SOLUTION
Plot the points and then connect the points with a smooth curve.
Direct Variation: the graph for this model is a line passing through the origin.
Direct
y = 2x
Inverse Variation: The graph for this model is a curve that gets closer and closer to the x-axis as x increases and closer and closer to the y-axis as x gets close to 0.
Inverse 8 x
y =

8. USING DIRECT AND INVERSE VARIATION IN REAL LIFE
Writing and Using a Model
BICYCLING A bicyclist tips the bicycle when making turn. The angle B of the bicycle from the vertical direction is called the banking angle.
banking angle, B

9. angle B and the turning radius r vary inversely.
Writing and Using a Model
BICYCLING The graph below shows a model for the relationship
between the banking angle and the turning radius for a bicycle traveling at a particular speed. For the values shown, the banking
angle B and the turning radius r vary inversely.
Turning Radius
Banking angle (degrees) r
turning radius
banking angle, B

10. Find an inverse variation model that relates B and r.
Writing and Using a Model
Turning Radius
Banking angle (degrees) r
turning radius
banking angle, B
Find an inverse variation model that relates B and r.
Use the model to find the banking angle for a turning radius of 5 feet.
Use the graph to describe how the banking angle changes as the turning radius gets smaller.

11. Writing and Using a Model
Turning Radius
Banking angle (degrees)
Find an inverse variation model that relates B and r.
SOLUTION
From the graph, you can see that B = 32° when r = 3.5 feet.
B = k r
Write direct variation model.
32 = k
3.5
Substitute 32 for B and 3.5 for r.
112 = k
Solve for k.
The model is B = , where B is in degrees and r is in feet.
112 r

12. Writing and Using a Model
Turning Radius
Banking angle (degrees)
Use the model to find the banking angle for a turning radius of 5 feet.
SOLUTION
Substitute 5 for r in the model you just found.
B =
112 5
= 22.4
When the turning radius is 5 feet, the banking angle is about 22°.

13. Writing and Using a Model
Use the graph to describe how the banking angle changes as the turning radius gets smaller.
SOLUTION
As the turning radius gets smaller, the banking angle becomes greater. The bicyclist leans at greater angles.
As the turning radius gets smaller, the banking angle becomes greater. The bicyclist leans at greater angles.
As the turning radius gets smaller, the banking angle becomes greater. The bicyclist leans at greater angles.
As the turning radius gets smaller, the banking angle becomes greater. The bicyclist leans at greater angles.
Notice that the increase in the banking angle becomes more rapid when the turning radius is small.