In mathematics (particularly in differential calculus), the derivative is a way to show instantaneous rate of change: that is, the amount by which a function is changing at one given point. For functions that act on the real numbers, it is the slope of the tangent line at a point on a graph. The derivative is often written as formula_1 ("dy over dx", meaning the difference in y divided by the difference in x). The "d" is not a variable, and therefore cannot be cancelled out. Another common notation is formula_2—the derivative of function formula_3 at point formula_4.