Calculus
Given a function . 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 , we know that if  is 1, then  will be 1; if  is 3, then  will be 9; if  is 20, then  will be 400. The slope of the tangent line produced using this method here is , or 2 multiplied by . So we know without having to draw any tangent line at any point on the curve  that the derivative, often written as  (marked with the prime symbol), will be  at any point. This process of working out a slope using limits is called differentiation, or finding the derivative.