c for cosine
o for opposite (?)
t for tangent
tangent = sine/cosine
An angle of a triangle, placed onto a unit circle, or onto cartesian system with vertex at the origin, and one of the sides along the x-axis will give but this is not enough. y would be the sine of the angle whose vertex is at the origin. Some length information is needed if y is not expected to be restricted to plus or minus 1.