Edexcel A Level Further Maths: Core Pure:复习笔记5.2.5 Integration by Substitution

The integrals covered in this revision note are based on the standard results

and

These are given in the formulae booklet

How do I know when to use a trigonometric substitution in integration?

There are three main types of problem

•  Type 1
  Showing the standard results using a substitution (α may have a value)
  The substitution will not be given in such cases
  e.g.  Use a suitable substitution to show that
  Let

The general idea in these types of problems is to reduce the denominator to a single term, often involving the identity , so it can be integrated using standard results or techniques

• Type 2
  Reverse chain rule, possibly involving some factorising in the denominator and using ‘adjust’ and ‘compensate’ if necessary
  e.g.  Find
  
• Type 3
  The denominator contains a three-term quadratic expression – i.e. there is an x term
  In such cases complete the square and use reverse chain rule
  e.g.  Find

• (This works since , so effectively there is no reverse chain rule involved)

• A fourth type of problem may involve a given substitution but the skills to solve these are covered in the A Level Mathematics course

How do I use a trigonometric substitution to find integrals?

• STEP 1
  Identify the type of problem and if a substitution is required
  Determine the substitution if needed
• STEP 2
  For Type 1 problems, differentiate and rearrange the substitution; change everything in the integral
  For Type 2 problems, ‘adjust’ and ‘compensate’ as necessary
  For Type 3 problems complete the square
• STEP 3
  Integrate using standard techniques and results, possibly from the formulae booklet
  For definite integration, a calculator may be used but look out for exact values being required, a calculator may give an approximation
• STEP 4
  Substitute the original variable back in if necessary – this shouldn’t be necessary for definite integration
  For indefinite integration, simplify where obvious and/or rearrange into a required format

Why is arccos x not involved in any of the integration results?

For integration the "-" at the start can be treated as the constant "-1" and so integrating would lead to "-arcsin ..."

• • i.e.