### Concept (1)

We often use the term direct variation to describe a form of dependence of one variable on another. An equation that makes a line and crosses the origin is a form of direct variation, where the magnitude of x increases or decreases directly as y increases or decreases. Direct variation and inverse variation are used often in science when modeling activity, such as speed or velocity.

### Sample Problems (5)

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If y varies directly with x and the value of y is 22 when the value of x is 2, find y when x = 5.

###### Problem 1
How to write the equation that relates two variables that vary directly.

A biker's distance traveled on flat terrain varies directly with its traveling time. If the biker has gone 90 miles after 2½ hours, how long will it take him to go 306 miles?

###### Problem 2
How to write the equation that relates two variables that vary directly from a word problem.
###### Problem 3
Using direct variation to write relationships for word problems and use the equation to make predictions
###### Problem 4
How to determine whether or not a relationship is direct variation or proportional
###### Problem 5
Using direct variation to find the constant of variation and write a direct variation equation, and use the equation to make predictions