# Thread: Trigo question. Help needed. Need to pass up on tomorrow.

1. ## Trigo question. Help needed. Need to pass up on tomorrow.

Given that $\cos x = -\frac{2}{3}$ and $\sin y = -\frac{1}{\sqrt 6}$, that $0^\circ\le x \le360^\circ$ and that $x$ and $y$ are in the same quadrant, find, without calculator, the values of $\cos\frac{x}{2}$.

Man. This question is killing me. I only get to know $x$ and $y$ are in quadrant 3.

2. Hi
Originally Posted by simonsong
Given that $\cos x = -\frac{2}{3}$ and $\sin y = -\frac{1}{\sqrt 6}$, that $0^\circ\le x \le360^\circ$ and that $x$ and $y$ are in the same quadrant, find, without calculator, the values of $\cos\frac{x}{2}$.

Man. This question is killing me. I only get to know $x$ and $y$ are in quadrant 3.
$\cos x=\cos \left(\frac{x}{2}+\frac{x}{2}\right)=2\cos^2\frac{ x}{2}-1$ hence $\cos \frac{x}{2}=\ldots$

This should give you two values for $\cos \frac{x}{2}$. As you also know in which quadrant is $\frac{x}{2}$, you can choose the right solution among the two you've found.

3. Omg. Never thought of that. Haha. Really thanks pal.