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受欢迎的三角函数 >

证明 2-2tan(x)cot(2x)=sec^2(x)

解答

证明

2 - 2 tan (x) cot (2 x) = sec^{2} (x)

解答

真

求解步骤

2 - 2 tan (x) cot (2 x) = sec^{2} (x)

调整左侧

2 - 2 tan (x) cot (2 x)

用 sin, cos 表示

2 - 2 cot (2 x) tan (x)

使用基本三角恒等式:

cot (x) = \frac{cos ( x )}{sin ( x )}

= 2 - 2 \cdot \frac{cos ( 2 x )}{sin ( 2 x )} tan (x)

使用基本三角恒等式:

tan (x) = \frac{sin ( x )}{cos ( x )}

= 2 - 2 \cdot \frac{cos ( 2 x )}{sin ( 2 x )} \cdot \frac{sin ( x )}{cos ( x )}

化简

2 - 2 \cdot \frac{cos ( 2 x )}{sin ( 2 x )} \cdot \frac{sin ( x )}{cos ( x )} : \frac{2 sin ( 2 x ) cos ( x ) - 2 cos ( 2 x ) sin ( x )}{sin ( 2 x ) cos ( x )}

2 - 2 \cdot \frac{cos ( 2 x )}{sin ( 2 x )} \cdot \frac{sin ( x )}{cos ( x )}

乘

2 \cdot \frac{cos ( 2 x )}{sin ( 2 x )} \cdot \frac{sin ( x )}{cos ( x )} : \frac{2 cos ( 2 x ) sin ( x )}{sin ( 2 x ) cos ( x )}

2 \cdot \frac{cos ( 2 x )}{sin ( 2 x )} \cdot \frac{sin ( x )}{cos ( x )}

分式相乘:

a \cdot \frac{b}{c} \cdot \frac{d}{e} = \frac{a \cdot b \cdot d}{c \cdot e}

= \frac{cos ( 2 x ) sin ( x ) \cdot 2}{sin ( 2 x ) cos ( x )}

= 2 - \frac{2 cos ( 2 x ) sin ( x )}{sin ( 2 x ) cos ( x )}

将项转换为分式:

2 = \frac{2 sin ( 2 x ) cos ( x )}{sin ( 2 x ) cos ( x )}

= \frac{2 sin ( 2 x ) cos ( x )}{sin ( 2 x ) cos ( x )} - \frac{cos ( 2 x ) sin ( x ) \cdot 2}{sin ( 2 x ) cos ( x )}

因为分母相等，所以合并分式:

\frac{a}{c} \pm \frac{b}{c} = \frac{a \pm b}{c}

= \frac{2 sin ( 2 x ) cos ( x ) - cos ( 2 x ) sin ( x ) \cdot 2}{sin ( 2 x ) cos ( x )}

= \frac{2 sin ( 2 x ) cos ( x ) - 2 cos ( 2 x ) sin ( x )}{sin ( 2 x ) cos ( x )}

= \frac{- 2 cos ( 2 x ) sin ( x ) + 2 cos ( x ) sin ( 2 x )}{cos ( x ) sin ( 2 x )}

使用三角恒等式改写

\frac{- 2 cos ( 2 x ) sin ( x ) + 2 cos ( x ) sin ( 2 x )}{cos ( x ) sin ( 2 x )}

使用倍角公式:

cos (2 x) = 2 cos^{2} (x) - 1

= \frac{- 2 ( 2 cos ^{2} ( x ) - 1 ) sin ( x ) + 2 cos ( x ) sin ( 2 x )}{cos ( x ) sin ( 2 x )}

使用倍角公式:

sin (2 x) = 2 sin (x) cos (x)

= \frac{- 2 ( 2 cos ^{2} ( x ) - 1 ) sin ( x ) + 2 cos ( x ) \cdot 2 sin ( x ) cos ( x )}{cos ( x ) \cdot 2 sin ( x ) cos ( x )}

化简

\frac{- 2 ( 2 cos ^{2} ( x ) - 1 ) sin ( x ) + 2 cos ( x ) \cdot 2 sin ( x ) cos ( x )}{cos ( x ) \cdot 2 sin ( x ) cos ( x )} : \frac{1}{cos ^{2} ( x )}

\frac{- 2 ( 2 cos ^{2} ( x ) - 1 ) sin ( x ) + 2 cos ( x ) \cdot 2 sin ( x ) cos ( x )}{cos ( x ) \cdot 2 sin ( x ) cos ( x )}

- 2 (2 cos^{2} (x) - 1) sin (x) + 2 cos (x) \cdot 2 sin (x) cos (x) = - 2 sin (x) (2 cos^{2} (x) - 1) + 4 cos^{2} (x) sin (x)

- 2 (2 cos^{2} (x) - 1) sin (x) + 2 cos (x) \cdot 2 sin (x) cos (x)

2 cos (x) \cdot 2 sin (x) cos (x) = 4 cos^{2} (x) sin (x)

2 cos (x) \cdot 2 sin (x) cos (x)

数字相乘:

2 \cdot 2 = 4

= 4 cos (x) sin (x) cos (x)

使用指数法则:

a^{b} \cdot a^{c} = a^{b + c}

cos (x) cos (x) = cos^{1 + 1} (x)

= 4 sin (x) cos^{1 + 1} (x)

数字相加:

1 + 1 = 2

= 4 sin (x) cos^{2} (x)

= - 2 sin (x) (2 cos^{2} (x) - 1) + 4 cos^{2} (x) sin (x)

= \frac{- 2 sin ( x ) ( 2 cos ^{2} ( x ) - 1 ) + 4 cos ^{2} ( x ) sin ( x )}{2 cos ( x ) sin ( x ) cos ( x )}

cos (x) \cdot 2 sin (x) cos (x) = 2 cos^{2} (x) sin (x)

cos (x) \cdot 2 sin (x) cos (x)

使用指数法则:

a^{b} \cdot a^{c} = a^{b + c}

cos (x) cos (x) = cos^{1 + 1} (x)

= 2 sin (x) cos^{1 + 1} (x)

数字相加:

1 + 1 = 2

= 2 sin (x) cos^{2} (x)

= \frac{- 2 sin ( x ) ( 2 cos ^{2} ( x ) - 1 ) + 4 cos ^{2} ( x ) sin ( x )}{2 cos ^{2} ( x ) sin ( x )}

分解

- 2 (2 cos^{2} (x) - 1) sin (x) + 4 sin (x) cos^{2} (x) : 2 sin (x)

- 2 (2 cos^{2} (x) - 1) sin (x) + 4 sin (x) cos^{2} (x)

改写为

= - 2 sin (x) (- 1 + cos^{2} (x) \cdot 2) + 2 \cdot 2 sin (x) cos^{2} (x)

因式分解出通项

2 sin (x)

= 2 sin (x) (- (- 1 + cos^{2} (x) \cdot 2) + 2 cos^{2} (x))

乘开

2 cos^{2} (x) - (2 cos^{2} (x) - 1) : 1

- (- 1 + cos^{2} (x) \cdot 2) + 2 cos^{2} (x)

= - (- 1 + 2 cos^{2} (x)) + 2 cos^{2} (x)

- (- 1 + cos^{2} (x) \cdot 2) : 1 - cos^{2} (x) \cdot 2

- (- 1 + cos^{2} (x) \cdot 2)

打开括号

= - (- 1) - (cos^{2} (x) \cdot 2)

使用加减运算法则

- (- a) = a, - (a) = - a

= 1 - cos^{2} (x) \cdot 2

= 1 - cos^{2} (x) \cdot 2 + 2 cos^{2} (x)

同类项相加:

- 2 cos^{2} (x) + 2 cos^{2} (x) = 0

= 1

= 2 \cdot 1 \cdot sin (x)

整理后得

= 2 sin (x)

= \frac{2 sin ( x )}{2 sin ( x ) cos ^{2} ( x )}

消掉

\frac{2 sin ( x )}{2 sin ( x ) cos ^{2} ( x )} : \frac{1}{cos ^{2} ( x )}

\frac{2 sin ( x )}{2 sin ( x ) cos ^{2} ( x )}

数字相除:

\frac{2}{2} = 1

= \frac{sin ( x )}{cos ^{2} ( x ) sin ( x )}

约分:

sin (x)

= \frac{1}{cos ^{2} ( x )}

= \frac{1}{cos ^{2} ( x )}

= \frac{1}{cos ^{2} ( x )}

= \frac{1}{cos ^{2} ( x )}

使用三角恒等式改写

使用基本三角恒等式:

cos (x) = \frac{1}{sec ( x )}

\frac{1}{( \frac{1}{s e c ( x )} ) ^{2}}

化简

\frac{1}{( \frac{1}{s e c ( x )} ) ^{2}}

(\frac{1}{sec ( x )})^{2} = \frac{1}{sec ^{2} ( x )}

(\frac{1}{sec ( x )})^{2}

使用指数法则:

(\frac{a}{b})^{c} = \frac{a ^{c}}{b ^{c}}

= \frac{1 ^{2}}{sec ^{2} ( x )}

使用法则

1^{a} = 1

1^{2} = 1

= \frac{1}{sec ^{2} ( x )}

= \frac{1}{\frac{1}{s e c ^{2} ( x )}}

使用分式法则:

\frac{1}{\frac{b}{c}} = \frac{c}{b}

= \frac{sec ^{2} ( x )}{1}

使用法则

\frac{a}{1} = a

= sec^{2} (x)

sec^{2} (x)

sec^{2} (x)

我们已展示，在两侧可以有相同的形式

\Rightarrow 真

流行的例子

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