Problem getting a Trig question check in My Calc

For example if I am given:

and is in

in quadrant III

I worked it out in by using the half angle formula and got

I usually use the old inverse trick on the calculator and check to see if the decimal I get matches the answer I got. But on several half angle problems I am getting an undefined message on my calc. What is the deal and how can I go about checking these...

Like what PerfectHacker did on this post http://www.mathhelpforum.com/math-he...i-89-calc.html

----------

Also one other question.... When I am using the double angle and half angle formulas, when I get what the single trig value is equal to such as and I am going to stick the value in a problem do I want to insert the negative or positive sign along with it depending on which quadrant I am talking about? For example in the problem above we are in the 3rd quadrant so both & both are negative. Would I want to insert them into the formulas as negative? But suppose we are in quadrant 1 where both & are positive. Would I want to put both values into the double or half-angle formulas as a postive?

I could use some real help with both questions if you could...

Thanks as always for the help!