A. In each of the following, y varies directly as x. Find the values as indicated.
1. If y = -18, when x = 9, find y when x = 7.
2. If y = 12 when x = 4, find y when x = 12.
3. If y = 20, when x = 4, find x when y = 80.
B. Write an equation where y varies directly as x.
- 1. y = 28 when x = 7
- 2. y = 48 when x = 6
Answers & Comments
A.
1. If y = -18, when x = 9, find y when x = 7.
y = kx
-18 = k9
-18 = 9k
-2 = k
k = y/x
-2 = y/7
7(2 = y/7)7
14 = y
y = 14
2. If y = 12 when x = 4, find y when x = 12.
y = kx
12 = k4
12= 4k
3= k
k = 3
k = y/x
3= y/12
12(2 = y/12)12
24 = y
y = 24
3. If y = 20, when x = 4, find x when y = 80.
y = kx
20 = k4
20 = 4k
5 = k
k = 5
5 = 80/x
-2 = y/7
x(5 = 80/x)x
5x = 80
y = 40
B
1. y = 28 when x = 7
y = kx
28 = k7
28 = 7k
4 = k
k = 4
2. y = 48 when x = 6
y = kx
48 = k6
48 = 6k
8 = k
k = 8