How to get a tangent line from a secant line

How to get slope of a tangent line from slope of secant line AB by moving B to A
[math]m_{tangent}=lim_{B\longrightarrow A}m_{AB}=lim_{B\rightarrow A}\frac{f\left(a+h\right)-f\left(a\right)}{h}[/math]
– Peiling Li

Information: How to get a tangent line from a secant line