# Trig Half-Angle Formula question

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• October 23rd 2010, 07:53 PM
mathmike2
Trig Half-Angle Formula question
Attachment 19431
Can anyone find my mistake in the above work. My book gives a different answer for the above question. The first line in the above is given by the book.
• October 23rd 2010, 11:07 PM
Prove It
For starters, in the region $\displaystyle \pi < \theta < \frac{3\pi}{2}$, you should know that $\sin{\theta} < 0,\cos{\theta} < 0$ and $\tan{\theta} > 0$.

So check your starting values of $\sin{\theta}$ and $\cos{\theta}$.