Today you will
Write equations and solve problems involving
Direct Variation
Inverse Variation
Joint Variation
3.8
Direct, Inverse, and Joint Variation
Direct Variation:
Inverse Variation:
Joint Variation:
The NHTSA reports an average braking distance of
227 feet for a car traveling 60 mph with a total stopping
distance of 359 feet.
Write an equation of direct variation relating braking
distance to car speed.
The braking distance varies directly as the square of the car's speed.
d
=
k
s2
227
=
k
(60)
2
b. Use your equation to find the braking distance of a car
traveling 70mph
HW Questions (# 1, 3, 5-7)
1. Describe each graph as illustrating a direct variation,
inverse variation or neither.
3. Explain why the graph does not represent a direct
variation.
Find the constant of variation for each relation and use it
to write an equation for each statement. Then solve the
equation to answer the question.
5. If y varies inversely as x and y = 3 when x = 4,
find y when x = 15
6. If y varies directly as the square of x and y = -54
when x = 9,
find y when x = 6
7. If y varies jointly as x and the cube of z
and y = 16 when x = 4 and z = 2,
find y when x = -8 and z = -3
Another example:
If y varies jointly as x and z and inversely as the square of w,
and y = 3 when x =3, z = 10, and w = 2
find y when x = 4, z = 20, and w = 4
HW:
3.8 (pg 194-195)
# 13-23 odds