The value of an inverse trig expression is an angle.
Inverse cosine is restricted to the angles in the first and second quadrant. That is, inverse cosine can only equal an angle
Since cosine is the x-coordinate of the point on the unit circle, you need to look for what point in the first or second quadrant on the unit circle has an x-coordinate of
Find the linear approximation of the function
Determine whether each of these functions is a bijection from R to R.