## What do Delta x / Delta y Mean?

The Greek letter Δ means “the change in” a variable. It’s usually used to mean a change in either the x-variable or y-variable:

**x**: a horizontal value on the x-axis.**Δx**: a horizontal change (from point “a” to point “b” on the x-axis)**y**: a vertical value on the y-axis.**Δy**: a vertical change (from point “a” to point “b” on the y-axis)

## Delta x / Delta y and The Slope

A linear (straight line) function has the equation

y = mx + b.

The “m” in the formula is the slope. In beginning algebra, you usually find the slope with the formula

**slope = rise over run.**

Where:

- Rise = change in y-values (from point a to point b).
- Run = change in x-values (from point a to point b).

## Delta x / Delta y and Derivatives

The notation Δ x is used in calculus (especially physics) to mean *tiny* changes or very small increments. These tiny changes (close to zero) allow us to find the derivative, which is the slope at a point.

A similar notation is dy/dx, which means “derivative”. However, there’s a big difference between Δy / Δx, and dy/dx:

- Δy / Δx, can be interpreted as meaning a
**quotient**of small differences in y and x. The quotient is the answer to a division problem [1]. For example, in the problem 10 / 5, 2 is the quotient.. - dy/dx means derivative and you can’t interpret it as a quotient of two numbers dy and dx.

The notation dy/dx is called Leibnitz notation. Another notation which means the same thing is f′ (prime notation), which avoids the confusion with a “quotient” [2].

Calculus suffers from a lack of consistent notation, partly due to so many different mathematicians working separately in the early years of its development. There are other various meanings for delta (both upper- and lower-case) including “discriminant”. See: Delta meaning.

## References

[1] Open University. 3.3.3 Multiplication and Division Math Terminology.

[2] Stony Brook Calculus Materials: Calculus I Handouts.