Integrating using Trigonometric Substitutions
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 ( may have a value)
The substitution will not be given in such cases
e.g. Use a suitable substitution to show that
Let , so and
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.
Examiner Tip
- The general form of the functions involving trigonometric and hyperbolic functions are very similar
- Be clear about which form needs a trigonometric substitution and which form need a hyperbolic substitution
- Always have a copy of the formula booklet to hand when practising these problems
Worked example
(a) Use an appropriate substitution to show that
(b) Find