Solving Trigonometric Equations

This page covers Solving Trigonometric Equations.

The various trigonometric formulae and identities can be used to help solve trigonometric equations. Here is a summary of the most important trigonometric formulae you should know:

sin2x + cos2x = 1

1 + cot2x = cosec2x

tan2x + 1 = sec2x

cos2x = cos2x - sin2x = 2cos2x - 1 = 1 - 2sin2x

sin2x = 2sinx cosx

tanx = sinx


The compound angle formulae: remember also the useful technique of writing expressions in the form rcos (q + a)

Proving Identities

You may be asked to use the formulae above to prove new trigonometric identities. To prove an identity, start from one side and manipulate it until you get the other side.


Prove that cosx cos2x + sinx sin2x = cosx

Starting from the left hand side:

cosx cos2x + sinx sin2x = cos(x - 2x)

= cos(-x) = cosx

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