What is “sin A” concretely ?
1. Draw a circle (diameter 1)
2. Connect any 3 points on the circle to form a triangle of angles A, B, C.
3. The length of sides opposite A, B, C are sin A, sin B, sin C, respectively.
By Sine Rule:
where sides a,b,c opposite angles A, B, C respectively.
a = sin A
b = sin B
c = sin C