Direct variation is a relation that has the form

y = kx

where k is the constant of proportionality.

If you are told that a relation is a direct proportion, and you are given one data point, you can find k. The you can write the equation of the direct relation.

Here is an example.
The price of gasoline follows a direct variation.
John bought 5 gallons of gas and paid $15.
a) Write an equation for the relation.
b) Using the relation you found, how much do 13.8 gallons cost?

Solution:
Since the relation is a direct variation, it follows the general equation of a direct variation:

y = kx

We are given one data point, 5 gallons cost $15.
We plug in 5 for x and 15 for y and we find k.

y = kx
15 = k * 5
k = 3

Now that we know that k = 3, we rewrite the relation using our value of k.

y = 3x

This is the answer to part a).

Part b)

We use our relation, y = 3x, and we plug in 13.8 into x and find y.

y = 3x

y = 3 * 13.8

y = 41.4

The price of 15 gallons of gas is $41.40.

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3 years ago