exercise:Cb67d8e6a0

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• Use theorem to write a formula for [math]\cos 2a[/math] in terms of [math]\cos a[/math] and [math]\sin a[/math].
• Similarly, use theorem to write a formula for [math]\sin 2a[/math].
• Write a formula for [math]\cos a[/math] and another for [math]\sin a[/math] in terms of [math]\cos \frac a2[/math] and [math]\sin \frac a2[/math].