Here's the question you clicked on:
babbyfromhell93
cos^-1(-sqrt 2/2)
check your Unit Circle if you don't have one -> http://i.stack.imgur.com/r8uHr.gif
cos^-1 (sqrt2/2) = inverse cos (sqrt2/2) think of an angle which has a cosine equal to (sqrt2/2) cos X = (sqrt2/2) X = 45 degrees, also pi/4 surely, 45 degrees is between -90 and 180 degrees hope that helped.