SOLUTION: Explain why {{{cos^(-1)(-0.9)}}} is not a negative number.

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Question 901450: Explain why cos%5E%28-1%29%28-0.9%29 is not a negative number.
Answer by Edwin McCravy(20056) About Me  (Show Source):
You can put this solution on YOUR website!
Since the cosine function is not one-to-one, its inverse would not be a
function.  So cos-1(x) was defined so that it would be a function by
restricting its domain to [0,p] or {y|0≦y≦p}, which is never negative.
Therefore by definition cos-1(-0.9) is between 0 and p, actually in 
quadrant II between pi%2F2 and p, approximately 2.69 (radians).
Edwin