Refers to the mapping between a set of arbitrary angles and a set of ratio variables. The usual trigonometric function is defined in the plane rectangular coordinate system, and its domain is the whole real number domain. The other is defined in a right triangle, but it is incomplete. Sinθ=y/r Sin θ = x/r cosθ=x/r tangent θ = y/x (where y indicates that the vertical side of a right triangle is the other vertical side and r is the hypotenuse).