Which of the following is an example of direct variation? y= cx
xy = c
y = czx
y= cx/z

Choice A) y = cx

The 'c' is the constant of variation

For example, if c = 2, then y = 2x is a direct variation. Whatever x is, we double it to get y. As x increases, so does y. As x decreases, then so does y. Both x and y increase/decrease together.

Direct variation equations always go through the origin, and they are always linear. The 'c' plays the role of the slope. You can think of y = cx as y = mx+b where b = 0 in this case and c = m.