# AQA A Level Maths: Pure复习笔记8.2.3 Reverse Chain Rule

### Reverse Chain Rule

#### What is the chain rule?

• The Chain Rule is a way of differentiating two (or more) functions

• In many simple cases the above formula/substitution is not needed

• The same can apply for the reverse – integration

#### Integrating with reverse chain rule

• In more awkward cases it can help to write the numbers in before integrating
• STEP 1: Spot the ‘main’ function
• STEP 2: ‘Adjust’ and ‘compensate’ any numbers/constants required in the integral
• STEP 3: Integrate and simplify

#### Exam Tip

• If in doubt you can always use a substitution.
• Differentiation is easier than integration so if stuck try the opposite, eg. sin and cos are linked (remember that minus!) so if integrating a sin function, start by differentiating the corresponding cos function.