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DEFINITION ON TRIANGLE:
Given a triangle with side length a (opposite to θ), b and hypotenuse c,
sin θ = a/c
cos θ = b/c
tan θ = sin θ / cos θ = a/b
DEFINITION ON CIRCLE:
Given a circle with radius r.
Draw a ray with angle θ from the centre.
The intersection of the line and the circle is (x,y)
sin θ = y/r
cos θ = x/r
tan θ = sin θ / cos θ = y/x
Note that θ is defined as 0 degrees when the ray coincides with the x-axis and has positive x. (right of origin)
θ varies from 0 to 360 degrees or 0 to 2pi (rad).