# Math Help - Trig Half-Angle Formula question

1. ## Trig Half-Angle Formula question

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.

2. 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 $\cos{\theta}$.