Given a function formula_1. "f" is short for function, so this equation means "y is a function of x". This tells us that how high y is on the vertical axis depends on what x (the horizontal axis) is at that time. For example, with the equation formula_2, we know that if formula_3 is 1, then formula_4 will be 1; if formula_3 is 3, then formula_4 will be 9; if "formula_3" is 20, then "formula_4" will be 400. The slope of the tangent line produced using this method here is formula_9, or 2 multiplied by "formula_3". So we know without having to draw any tangent line at any point on the curve formula_11 that the derivative, often written as formula_12 (marked with the prime symbol), will be formula_9 at any point. This process of working out a slope using limits is called differentiation, or finding the derivative.