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An identity is an equation which is true for all values of the variable.
There are many trig identities that are useful for changing the appearance of an expression.
The most important ones should be committed to memory.
The general method of proving an identity is to choose the left-hand side and show, by using known identities, that it can be converted into the form on the right-hand side.
Trig 5 PowerPoint File 0.6Mb
Trig Identity Worksheet Word File 34 Kb