三角学中,这么一堆公式其实就说了2个事而已

三角学中,这么一堆公式其实就说了”2个事” 而已: – 1, i

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Dr. Adrian Yeo Ning Hong’s Math Books

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 !!!
Yes!
He has wonderful tricks to make such difficult Secondary 3 Math easy for kids, and of course, also helps the weak Math teenagers.

Interesting !
Trig Or Treat: An Encyclopedia of Trigonometric Identity Proofs With Intellectually Challenging Games

The other books by him are:

The Pleasures of Pi,e and Other Interesting Numbers

What is “sin A”

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:

\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

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