Question

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Answer 1

A direct variation is in the form of y=k•x
where y is proportional to x, and k is a proportion constant.

Answer 2

If you have,
y=k•x+b
(when you add (or subtract) some number (which I called it b),
then your x and y are not proportional.)

Answer 3

You can also think of ot this way,
A direct variation is a line that goes through the origin (0,0).