Odds will change but Evens are conserved, Notations get depend on where they are：奇变偶不变，符号看象限。
一、歌词
When you first study math about 1,2,3,4
当你初学数学中的1,2,3,4
First study equation about X,Y,Z,T
初学方程中的 X,Y,Z,T
It will help you to think in a logical way
它将帮助你进行逻辑思考
When you sing sine, cosine, cosine, tangent
当你唱起正弦，余弦，余弦，正切
Sine, cosine, tangent, cotangent
正弦，余弦，正切，余切
Sine, cosine, secant, cosecant
正弦，余弦，正割，余割
Let's sing a song about trig-functions
让我们唱起三角函数的歌谣吧
（一）化简一（上世纪被错翻译为“诱导”）
Which is induction formula 1, and induction formula 2
这是诱导公式归类1，下面是诱导公式归类2
（二）化简二
These are all those "name don't change"
这些均为“函数名不变”
As pi goes to half pi the difference shall be huge
当π成为\dfrac{π}{2}是变化会很大
That is to say the odds will change, evens are conserved
这就是说 ：奇变偶不变
The notations that they get depend on where they are
符号看象限
But no matter where you are
但不论你在哪
I've gotta say that
我将会说
If you were my sine curve, I'd be your cosine curve
你若为正弦曲线，我愿做余弦曲线
I'll be your derivative, you'll be my negative one
我将为你的导数，你将为我负导数
As you change you amplitude, I change my phase
当你改变振幅，我改变相位
We can oscillate freely in the external space
我们可在外界空间自由震荡
As we change our period and costant at hand
当我们改变周期和手边常数
We travel from the origin to infinity
我们从原点驶向无尽
It's you sine, and you cosine
是你，正弦，余弦
Who make charming music around the world
创造了世间动人的音乐
It's you tangent, cotangent
是你，正切，余切
Who proclaim the true meaning of centrosymmetry
揭示了中心对称的真谛
B BOX……
You wanna measure width of a river, height of a tower
你想测量河宽及塔高
You scratch your head which cost you more than an hour
你抓耳挠腮一个多小时也想不出
You don't need to ask any "gods" or" master" for help
你无需向神或大佬们请教
This group of formulas are gonna help you solve
这一组公式将帮你解决
（三）两角和差
As you come across a right triangle you fell easy to solve
当你遇到直角三角形很容易解
But an obtuse trianges gonna make you feel confused
但钝角三角形使你感到困惑
Don't worry about what you do
无须担心
There are always means to solve
总有解决方法
As long as you master the sine cosine law
只要你掌握了正余弦定理
At this moment I've got nothing to say
此刻我无以言表
As trig-functions rain down upon me
当时三角函数犹雨点般落向我
At this moment I've got nothing to say
此刻我无以言表
Let's sing a song about trig-functions
让我们唱起三角函数歌谣吧
Long live the trigonometric functions
三角函数万岁
二、其余公式
（四）正余弦公式
1. 正弦
2. 余弦
(1) 标准公式
（当\gamma为90°时，cos\gamma=0，余弦定理可简化为c^2=a^2+b^2，即勾股定理）