U SING D IRECT AND I NVERSE V ARIATION D IRECT V ARIATION The variables x and y vary directly if, for a constant k, y x =k,k, k  0. or y = kx,

U SING D IRECT AND I NVERSE V ARIATION I NDIRECT V ARIATION The variables x and y vary inversely, if for a constant k, k x =y,y, k  0. or xy = k,

U SING D IRECT AND I NVERSE V ARIATION M ODELS FOR D IRECT AND I NVERSE V ARIATION D IRECT V ARIATION I NVERSE V ARIATION y = kx k > 0 k x = y

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

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

Comparing Direct and Inverse Variation S OLUTION Inverse Variation: k > 0. As x doubles (from 1 to 2), y is halved (from 8 to 4). 8 x y =y = x y = 2x 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 Direct Variation: k > 0. As x increases by 1, y increases by 2.

S OLUTION Compare the direct variation model and the inverse variation model you just found using x = 1, 2, 3, and 4. Comparing Direct and Inverse Variation Plot the points and then connect the points with a smooth curve. 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. Direct Variation: the graph for this model is a line passing through the origin. Direct y = 2x Inverse 8 x y =y =

B ICYCLING A bicyclist tips the bicycle when making turn. The angle B of the bicycle from the vertical direction is called the banking angle. U SING D IRECT AND I NVERSE V ARIATION IN R EAL L IFE Writing and Using a Model banking angle, B

B ICYCLING 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. Writing and Using a Model r turning radius banking angle, B Turning Radius Banking angle (degrees)

Turning Radius Banking angle (degrees) Writing and Using a Model 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. r turning radius banking angle, B

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

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

Writing and Using a Model Use the graph to describe how the banking angle changes as the turning radius gets smaller. S OLUTION 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. As the turning radius gets smaller, the banking angle becomes greater. The bicyclist leans at greater angles.