# Trigonometric substitution

### Trigonometric substitution

In this section, we will look at evaluating trigonometric functions with trigonometric substitution. A lot of people normally substitute using trig identities, which you will have to memorize. However, Dennis will use a different and easier approach. He will use the idea of Pythagoras Theorem and make a relation with sine and cosine in terms of x to substitute. In more advance questions, we will use Pythagoras Theorem to make a relation with tangent and secant to substitute. No matter what type of questions you do, you will only need to know Pythagoras Theorem and nothing else!