Triginometry

Things to already know:
Equation for a circle.
radius of a unit circle.
How to select the sign for Y to place the answer into the right quadrant.

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We know:
X = -4/5, r = 1
Equation for a circle: r^2 = X^2 + Y^2
Solving for the unknown Y: Y = +/- sqrt(r^2 - X^2)
if r > X, then we have a real answer.

Once the square root is computed one has to select the sign to place point into the 2nd quadrant.

(-4/5, y) = (cos theta, sin theta)
------> h = 5, a = 4, o = 3
Because it's in QII ----> sin > 0
So sin theta = + 3/5 = y