calculator issue: radians or degrees for inverse trig It's a simple question but I am a little confused.  The value of $cos^{-1} (-0.5)$ ,  is it   2.0943 or 120  ? 

It helps to understand that there are several different functions called cosine. I find it useful to refer to "cos" (the thing for which $\cos^{-1}(0) = \pi/2$) and "cosd" for which cosd(90) = 0. 
Your calculator (if you're lucky) will mean "cos" when you press the "cos"  button; if you've got the option of "degrees/radians", then in "degrees" mode, the "cos" button is actually computing the function "cosd". 
