# Proving Trigonometry Identities Help

• Jan 11th 2009, 11:19 PM
fayeorwhatsoever
Proving Trigonometry Identities Help
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• Jan 11th 2009, 11:58 PM
Isomorphism
Quote:

This question shows you are not familiar with the subject. My advice is that you should read your textbook/notes again. Work out few simple problems.

By the way -tan x -tan x = -2 tan x

I will answer your question, but I doubt you will understand every step clearly. First read the identities and know them clearly and then re read my post, you will understand the solution.

Quote:

Originally Posted by fayeorwhatsoever
Prove: sin squared 2x cos squared 2x=1/8 (1-cos8x)

$\sin ^2 2x \cos^2 2x = \frac{\sin^2 4x}{4} = \frac14\left(\frac{1 - \cos 8x}{2}\right) = \frac18 (1 - \cos 8x)$

For the first equality, I have used the relation $\sin 2y = 2 \sin y \cos y$

For the second equality I have used $\cos 2y = 1 - 2\sin^2 y$