x = cost
y = sint
Consider a right triangle with t as one of the acute angles.
cost = x = x/1 --> the adjacent leg is x, the hypotonuse is 1
sint = y = y/1 --> the oposite leg is y, the hypotouse is 1
from this we can use the pythagorean theorm to find x and y:
a^2 + b^2 = c^2
x^2 + y^2 = 1^2 = 1
That's how we relate x and y. doing it the other way (solving for t) won't work out very well in this situation.