# 三角学中，这么一堆公式其实就说了2个事而已

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Video:

# Trigonometry Formula Memory Trick

Draw picture (like below) to remember for life:
cosine & sine of ϴ & (π/2 – ϴ)

# Right Way To Teach Trigonometry

Few teachers in the world ever teach the “sine” function as defined originally by the angle (θ = 1 radian) formed by the chord (弦) of length 1 in an unit circle ( of radius 1).

Chinese truthfully translated in the original definition as shown in the picture.

sin 正弦: “Right Chord”

cos 馀弦 “Remaining Chord”

tan 正切 “Right Cut”:

# 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.

The other books by him are:

# Beautiful Trigonometry

Beautiful Trigonometry
tan α +tan β +tan γ = tan α.tan β.tan γ

# 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