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.
Proof:
By Sine Rule:
$latex frac{a}{sin A} = frac{b}{sin B} =frac{c}{sin C} = 2R = 1$
where sides a,b,c opposite angles A, B, C respectively.
a = sin A
b = sin B
c = sin C
Mathtuition88 