# BGN-2.1.1

For English version, see here and here and here and under the third title.

# 定义

$\sin A=\dfrac{a(O)}{c(H)}(SOH) ,\cos A=\dfrac{b(A)}{c(H)}(CAH) ,\tan A=\dfrac{a(O)}{b(A)}(TOA) .(SOH~CAH~TOA)$ Image 2.1.1-1 图2.1.1-2 而角 $A$ 的余切，正割，余割则记作 $\cot A,\sec A,\csc A$ ，定义为 $\cot A=\dfrac{1}{\tan A},\sec A=\dfrac{1}{\cos A},\csc A=\dfrac{1}{\sin A}$

# Special values 特殊值

See the table below. 见下表。

 $\sin$ $\cos$ $\tan$ $30^\circ$ $\frac{1}{2}$ $\frac{\sqrt{3}}{2}$ $\frac{1}{\sqrt{3}}$ $45^\circ$ $\frac{\sqrt{2}}{2}$ $\frac{\sqrt{2}}{2}$ $1$ $60^\circ$ $\frac{\sqrt{3}}{2}$ $\frac{1}{2}$ $\sqrt{3}$

# Question 习题

Given that $\cos (30^\circ )=\frac{\sqrt{3}}{2}$, find $\csc (60^\circ )$.

Note by Jeff Giff
5 months, 2 weeks ago

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