 Geometrically, the derivative at a point is the slope of the tangent line to the graph of the function at that point, provided that the derivative exists and is defined at that point.For a real-valued function of a single real variable, the derivative of a function at a point generally determines the best linear approximation to the function at that point.Differential calculus and integral calculus are connected by the fundamental theorem of calculus, which states that differentiation is the reverse process to integration.