Determine exact value of cos(cos^-1(19 pi)).

Question

is this the cos (a+b)= cos a cos b- sina sin b? or is it something different. When plugging it in the calculator, do we enter it with cos and then the (cos^-1(19 pi)).

Answer 1

an even number of pi is all the way around,360 degrees
and odd number of pi leaves you at 180, the negative x axis
the cos there at (-1,0) on theta = 180 degrees is -1