Use of double angle formulae It's good to know that to solve any trigonometric equation involving \ (\sin 2x\) and either \ (\sin x\) or \ (\cos x\), the process is the same.

Find the exact value of cos 2 x. Use Pythagoras' theorem to work out the hypotenuse, giving you sin x = 13 7 and cos x = 6 7. You can use any of the three formulae for cos 2 x.