2.3 Direct Variation
September 17, 2008
2.3 Direct Variation
Objective
Write and interpret
direct variation equations.
Old paper
1
2.3 Direct Variation
September 17, 2008
Check Skills You'll Need:
Solve each equation for y:
1. 12y = 3x
2. 12y = 5x
3. (3/4)y = 15
4. 0.9y = 27x
Tell whether each equation is true:
5.
1
4
=
2
8
6.
2
5
=
6
15
7.
9
24
=
12
36
8.
20
24
=
30
36
Jul 31­8:41 AM
2
2.3 Direct Variation
September 17, 2008
A DIRECT VARIATION
is a line that always goes through the origin.
y = mx + b
slope
What will ALWAYS be
the value of b?
y­intercept
Jul 31­9:02 AM
3
2.3 Direct Variation
September 17, 2008
The general form of a direct variation is
y = kx k is the variation constant
How do we find k?
y = kx
Divide both sides by x.
y
k= x
Jul 31­9:14 AM
4
2.3 Direct Variation
September 17, 2008
For the following function, does y vary directly with x?
If so, find the constant of variation and write the equation.
x y
2 8
3 12
5 20
k=4
y = 4x
Jul 31­9:36 AM
5
2.3 Direct Variation
September 17, 2008
For the following function, does y vary directly with x?
If so, find the constant of variation and write the equation.
x y
1 4
2 7
5 16
y/x is not constant
y does not vary directly with x
Jul 31­9:43 AM
6
2.3 Direct Variation
September 17, 2008
Can you identify a direct variation from an equation?
For each function, determine whether y varies directly with x.
If so, find the constant of variation.
a. 3y = 2x
b. y = 2x + 3
y = (2/3)x
k = 2/3
not direct
variation
Jul 31­9:47 AM
7
2.3 Direct Variation
September 17, 2008
A dripping faucet wastes a cup of water if it drips for three minutes. The amount
of water wasted varies directly with the amount of time the faucet drips. Find the
constant of variation k and write an equation to model the direct variation.
Step 1: What is the problem asking us?
water wasted varies directly
with time
Step 2: Define variables.
Let w = # of cups wasted
Let t = time in minutes the faucet drips
Step 3: Write an equation.
w = kt
1 = k(3)
1/3 = k
Substitute 1 for w and 3 for t.
Step 4: Write the direct variation equation with k.
w = (1/3)t
Jul 31­10:25 AM
8
2.3 Direct Variation
September 17, 2008
y varies directly with x
x = 27 and y = -51
Find x when y = -17
Step 1: Set up a proportion
y1
y2
=
x1
x2
Step 2: Plug in the numbers.
­51
­17
=
x2
27
Step 3: Simplify.
x2 = 9
Jul 31­11:21 AM
9
2.3 Direct Variation
September 17, 2008
HOMEWORK
p. 74 #1 ­ 27, 52
Old paper
10