11.01 Directly proportional functions

Lesson

Worksheet

Practice

Lesson

Now we are going to look at a special kind of linear relationship called a proportional relationship.

Two quantities are said to be proportional if they vary in such a way that one is a constant multiple of the other. In other words, they always vary by the same constant.

For example, if you earn $\$18$$18 per hour, your earnings are directly proportional to the number of hours worked because $\text{earnings }=18\times\text{hours worked }$earnings =18×hours worked .

The equation for this relationship can be written as $y=18x$y=18x, where $y$y is the total earnings and $x$x is the number of hours worked.

The constant of proportionality

The constant of proportionality is the value that relates the two amounts. In the example above, the constant would be $18$18. Notice that this is exactly the same as saying the gradient of the line is $18$18. A directly proportional relationship is a linear equation that has $c=0$c=0. In other words, it passes through the origin.

We can write a general equation for amounts that are directly proportional.

Equation for directly proportional relationships

$y=kx$y=kx

where $k$k is the constant of proportionality.

Once we solve the constant of proportionality, we can use it to answer other questions in this relationship.

Did you know?

Whilst the constant of proportionality can be any non-zero number, positive or negative, we generally only consider relationships with a positive value of k. That is, those with a positive rate of change.

Worked examples

Question 1

Consider the values in each table. Which of them could represent a directly proportional relationship between $x$x and $y$y?

$x$x $1$1 $3$3 $5$5 $7$7

$y$y $20$20 $16$16 $12$12 $8$8

A

$x$x $1$1 $5$5 $6$6 $20$20

$y$y $16$16 $12$12 $8$8 $4$4

B

$x$x $1$1 $2$2 $3$3 $4$4

$y$y $2$2 $8$8 $18$18 $32$32

C

$x$x $1$1 $2$2 $3$3 $4$4

$y$y $2$2 $4$4 $6$6 $8$8

D

$x$`x`	$1$1	$3$3	$5$5	$7$7
$y$`y`	$20$20	$16$16	$12$12	$8$8

$x$`x`	$1$1	$5$5	$6$6	$20$20
$y$`y`	$16$16	$12$12	$8$8	$4$4

$x$`x`	$1$1	$2$2	$3$3	$4$4
$y$`y`	$2$2	$8$8	$18$18	$32$32

$x$`x`	$1$1	$2$2	$3$3	$4$4
$y$`y`	$2$2	$4$4	$6$6	$8$8

Question 2

Consider the equation $P=70t$P=70t.

State the constant of proportionality.
Find the value of $P$P when $t=4$t=4.

Question 3

$William$ is making cups of fruit smoothie. The amount of bananas and strawberries he uses is shown in the proportion table.

Strawberries	$5$5	$10$10	$15$15	$20$20	$25$25
Bananas	$3.5$3.5	$7$7	$10.5$10.5	$14$14	$17.5$17.5

Sketch the graph of this proportional relationship.

Loading Graph...
What is the unit rate of this relationship?
Select ALL the statements that describe the proportional relationship.
For every $3.5$3.5 bananas $William$ uses, he adds $5$5 strawberries.
A
For every $5$5 bananas, $William$ uses $3.5$3.5 strawberries.
B
The unit rate of bananas in respect to strawberries is $\frac{7}{10}$710.
C
The unit rate of bananas in respect to strawberries is$\frac{10}{7}$107.
D