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Two variables are directly proportional when their ratio stays constant as their values change. You can check this by dividing one variable by the other and seeing whether the same number is produced each time. The directly proportional symbol ∝ is used to show this relationship, such as y ∝ x, which means that y changes in direct relation to x. Another useful clue is that multiplying one variable by a certain factor causes the other variable to change by that same factor.