What Is Direct Proportion?
Definition
Direct Proportion: A relationship in which one quantity grows or shrinks by the same factor as another. The ratio of matching values stays constant.
Direct proportion is a relationship in which one quantity grows or shrinks by the same factor as another. The ratio of matching values stays constant: k = b / a. For example, a car that covers 300 km in 4 hours travels 75 km per hour and covers 525 km in 7 hours at the same speed.
Where it appears in the formulas
In a direct proportion a / b = c / x, so x = b × c / a and the constant is k = b / a. For example, if 2 kg of a product cost 150, then 5 kg cost x = 150 × 5 / 2 = 375; the price per kilogram is k = 75.
How it differs from inverse proportion
In a direct proportion the ratio of matching values stays constant; in an inverse proportion their product does. When one quantity doubles, the other doubles in a direct proportion and halves in an inverse one.