## 选项

[A].   $\cos ^{2} \alpha =$ $\frac{1 + \cos \alpha}{2}$
[B].   $\cos ^{2} \alpha =$ $\frac{1 – \cos 2 \alpha}{2}$
[C].   $\cos ^{2} \alpha =$ $\frac{1 + \cos 2 \alpha}{2}$
[D].   $\cos ^{2} \alpha =$ $\frac{2 + \cos 2 \alpha}{2}$

$\cos ^{2} \alpha =$ $\frac{1 + \cos 2 \alpha}{2}$

## 选项

[A].   $\cot 2 \alpha =$ $\frac{\cot ^{2} \alpha + 1}{2 \cot \alpha}$
[B].   $\cot 2 \alpha =$ $\frac{\cot ^{2} \alpha – 1}{2 \cot \alpha}$
[C].   $\cot 2 \alpha =$ $\frac{2 \cot \alpha}{\cot ^{2} \alpha – 1}$
[D].   $\cot 2 \alpha =$ $\frac{\cot ^{2} \alpha – 2}{2 \cot \alpha}$

$\cot 2 \alpha =$ $\frac{\cot ^{2} \alpha – 1}{2 \cot \alpha}$

## 选项

[A].   $\tan 2 \alpha =$ $\frac{2 \tan \alpha}{1 + \tan ^{2} \alpha}$
[B].   $\tan 2 \alpha =$ $\frac{1 – \tan ^{2} \alpha}{2 \tan \alpha}$
[C].   $\tan 2 \alpha =$ $\frac{\tan \alpha}{1 – \tan ^{2} \alpha}$
[D].   $\tan 2 \alpha =$ $\frac{2 \tan \alpha}{1 – \tan ^{2} \alpha}$

$\tan 2 \alpha =$ $\frac{2 \tan \alpha}{1 – \tan ^{2} \alpha}$

## 选项

[A].   $\sin ^{2} \alpha =$ $\frac{1 – \cos \alpha}{2}$
[B].   $\sin ^{2} \alpha =$ $\frac{2 – \cos 2 \alpha}{2}$
[C].   $\sin ^{2} \alpha =$ $\frac{1 + \cos 2 \alpha}{2}$
[D].   $\sin ^{2} \alpha =$ $\frac{1 – \cos 2 \alpha}{2}$

$\sin ^{2} \alpha =$ $\frac{1 – \cos 2 \alpha}{2}$

## 选项

[A].   $\cos 2 \alpha =$ $\cos ^{2} \alpha -$ $1$
[B].   $\cos 2 \alpha =$ $2 \cos ^{2} \alpha +$ $1$
[C].   $\cos 2 \alpha =$ $2 \cos ^{2} \alpha -$ $1$
[D].   $\cos 2 \alpha =$ $\cos ^{2} \alpha +$ $1$

$\cos 2 \alpha =$ $2 \cos ^{2} \alpha -$ $1$

## 选项

[A].   $\cos 2 \alpha =$ $2 -$ $2 \sin ^{2} \alpha$
[B].   $\cos 2 \alpha =$ $1 +$ $2 \sin ^{2} \alpha$
[C].   $\cos 2 \alpha =$ $1 -$ $2 \sin ^{2} \alpha$
[D].   $\cos 2 \alpha =$ $1 -$ $\sin ^{2} \alpha$

$\cos 2 \alpha =$ $1 -$ $2 \sin ^{2} \alpha$

## 选项

[A].   $\cos 2 \alpha =$ $\sin ^{2} \alpha -$ $\cos ^{2} \alpha$
[B].   $\cos 2 \alpha =$ $- \cos ^{2} \alpha -$ $\sin ^{2} \alpha$
[C].   $\cos 2 \alpha =$ $\cos ^{2} \alpha +$ $\sin ^{2} \alpha$
[D].   $\cos 2 \alpha =$ $\cos ^{2} \alpha -$ $\sin ^{2} \alpha$

$\cos 2 \alpha =$ $\cos ^{2} \alpha -$ $\sin ^{2} \alpha$

## 选项

[A].   $\sin 2 \alpha =$ $2 \sin \alpha \cos \alpha$
[B].   $\sin 2 \alpha =$ $2 \sin \alpha \csc \alpha$
[C].   $\sin 2 \alpha =$ $\frac{1}{2} \sin \alpha \cos \alpha$
[D].   $\sin 2 \alpha =$ $\sin \alpha \cos \alpha$

$\sin 2 \alpha =$ $2 \sin \alpha \cos \alpha$

## 选项

[A].   $1 -$ $\csc ^{2} \alpha =$ $\cot ^{2} \alpha$
[B].   $1 +$ $\csc ^{2} \alpha =$ $\cot ^{2} \alpha$
[C].   $1 -$ $\cot ^{2} \alpha =$ $\csc ^{2} \alpha$
[D].   $1 +$ $\cot ^{2} \alpha =$ $\csc ^{2} \alpha$

$1 +$ $\cot ^{2} \alpha =$ $\csc ^{2} \alpha$

## 选项

[A].   $1 -$ $\tan ^{2} \alpha =$ $\sec ^{2} \alpha$
[B].   $1 +$ $\tan ^{2} \alpha =$ $\sec ^{2} \alpha$
[C].   $1 +$ $\sec ^{2} \alpha =$ $\tan ^{2} \alpha$
[D].   $1 +$ $\tan ^{2} \alpha =$ $- \sec ^{2} \alpha$

$1 +$ $\tan ^{2} \alpha =$ $\sec ^{2} \alpha$

## 选项

[A].   $\sin ^{2} \alpha -$ $\cos ^{2} \alpha =$ $0$
[B].   $\sin ^{2} \alpha +$ $\cos ^{2} \alpha =$ $-1$
[C].   $\sin ^{2} \alpha +$ $\cos ^{2} \alpha =$ $1$
[D].   $\sin ^{2} \alpha -$ $\cos ^{2} \alpha =$ $1$

$\sin ^{2} \alpha +$ $\cos ^{2} \alpha =$ $1$

## 选项

[A].   $\cot \alpha =$ $\frac{1}{\sin \alpha}$
[B].   $\cot \alpha =$ $\frac{\sin \alpha}{\cos \alpha}$
[C].   $\cot \alpha =$ $\frac{\cos \alpha}{\sin \alpha}$
[D].   $\cot \alpha =$ $\frac{1}{\cos \alpha}$

$\cot \alpha =$ $\frac{\cos \alpha}{\sin \alpha}$

## 选项

[A].   $\tan \alpha =$ $\frac{1}{\sin \alpha}$
[B].   $\tan \alpha =$ $\frac{1}{\cos \alpha}$
[C].   $\tan \alpha =$ $\frac{\cos \alpha}{\sin \alpha}$
[D].   $\tan \alpha =$ $\frac{\sin \alpha}{\cos \alpha}$

$\tan \alpha =$ $\frac{\sin \alpha}{\cos \alpha}$

## 选项

[A].   $\begin{cases} & \tan \alpha \cdot \cot \alpha = 1; \\ & \sin \alpha \cdot \sec \alpha = 1; \\ & \cos \alpha \cdot \csc \alpha = 1. \end{cases}$
[B].   $\begin{cases} & \tan \alpha \cdot \cot \alpha = -1; \\ & \sin \alpha \cdot \csc \alpha = -1; \\ & \cos \alpha \cdot \sec \alpha = -1. \end{cases}$
[C].   $\begin{cases} & \tan \alpha \cdot \cot \alpha = 1; \\ & \sin \alpha \cdot \csc \alpha = 1; \\ & \cos \alpha \cdot \sec \alpha = 1. \end{cases}$
[D].   $\begin{cases} & \tan \alpha \cdot \csc \alpha = 1; \\ & \sin \alpha \cdot \cot \alpha = 1; \\ & \cos \alpha \cdot \sec \alpha = 1. \end{cases}$

$\begin{cases} & \tan \alpha \cdot \cot \alpha = 1; \\ & \sin \alpha \cdot \csc \alpha = 1; \\ & \cos \alpha \cdot \sec \alpha = 1. \end{cases}$

## 选项

[A].   $\csc A =$ $\frac{b}{c}$
[B].   $\csc A =$ $\frac{b}{a}$
[C].   $\csc A =$ $\frac{a}{b}$
[D].   $\csc A =$ $\frac{b}{c}$

$\csc A =$ $\frac{b}{a}$