Interestingly enough, the trigonometric functions that define those relationships are also closely tied to circles. Needless to say, this makes trig one of the hardest topics in math for students ...

There are three main trigonometric functions - Sine θ (sin θ), Cosine θ (cos θ) and Tangent θ (tan θ). Sin θ - Ratio of Perpendicular to Hypotenuse Cos θ - Ratio of Base to Hypotenuse Tan ...