Dr. Yeo Ning Hong was the former cabinet Minister of Singapore. He wrote a few Math books after retirement to teach his grand-daughters in primary schools – on Trigonometric Identity Proofs !!!
He has wonderful tricks to make such difficult Secondary 3 Math easy for kids, and of course, also helps the weak Math teenagers.
The other books by him are:
tan α +tan β +tan γ = tan α.tan β.tan γ
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