What is the chain rule?

The chain rule is a type of differentiation used when you are differentiating a function of a function e.g. (2x+5)^3, or e.g.2. (x^2 + 5)^0.5.
Firstly you replace one function with u. You then differentiate to produce dy/du in the normal way. Then do the same to produce du/dx. To get dy/dx you must do dy/du multiplied by du/dx and replace any u's with the acutal formula. For example, if y=(2x+5)^3, then let u=2x+5. dy/du=3u^2. du/dx=2. Thus, dy/dx=2 x 3u^2 =6u^2 =6(2x+5)^2.