Hi
here in like thi9s queation we use ( Rad ) or (deg ) in the calcolator ?
Consider the angle $\displaystyle \theta = \arccos\left(\frac{-\sqrt{3}}{2}\right)$
Look at the $\displaystyle 30-60-90$ triangle here Special right triangles - Wikipedia, the free encyclopedia
Do you notice anything about the side lengths in relation to the problem?
Now map this onto this circle for where $\displaystyle \cos$ is negative. Have a read of this. http://www.haverford.edu/physics/Mat...Trig_Ident.pdf