# Direct proportionality

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Two quantities **a** and **b** are said to be **directly proportional**, if the quotient is constant. This is written as .

## Constant of Proportionality

The constant is called *constant of proportionality*. Therefore a *proportionality* can be written as:

## Famous Proportionality

- The circumference
**C**of a circle is proportional to its radius**r**:

- Here, the constant is

- Mass–energy equivalence: The energy
**E**of a body is proportional to its mass**m**:

- Here, the constant is
**c²**, the square of the speed of light.

## Misunderstandings

The role of the quantities and of the constant are not interchangeable. It would be wrong to say in the examples above that or .

## Sources

Weisstein, Eric W. "Proportional." From MathWorld--A Wolfram Web Resource.