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# Trigonometry question c3 plz help watch

1. Prove that
& = theta
1-tan^2 &
_________ = cos 2&
1+tan^2 &
2. Uh, I don't think that came out as intended.. or I don't understand the question.
3. Ill use t ... I think.

1-tan^2 t / 1+tan^2 t

Invoke, tan^2 t + 1 = sec^2 t

1-tan^2 t / 1+(sec^2 t - 1)

1-tan^2 t (cos^2 t) (as 1/sec^2 t is cos^2 t)

cos^2 t - sin^2 t

This is a standard identitiy and it equals, cos 2t.

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Updated: April 14, 2006
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