The tangent line is the line that intesects a function f at a single point P.
(PQ) is secant to the curve f in two points, P and Q.
Its equation is expressed us:
Thus (PQ) will be tangent to the curve f when P and Q coincide.
That is when h tend to 0.
Therefore, the tangent or derivative f´(x) is the instanteneous rate of change.
Mathematically expressed as: