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受欢迎的三角函数 >

证明 1/2 cot(x)-1/2 tan(x)=cot(2x)

解答

证明

\frac{1}{2} cot (x) - \frac{1}{2} tan (x) = cot (2 x)

解答

真

求解步骤

\frac{1}{2} cot (x) - \frac{1}{2} tan (x) = cot (2 x)

调整左侧

\frac{1}{2} cot (x) - \frac{1}{2} tan (x)

用 sin, cos 表示

cot (x) \frac{1}{2} - \frac{1}{2} tan (x)

使用基本三角恒等式:

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

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

使用基本三角恒等式:

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

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

化简

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

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

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

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

分式相乘:

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

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

乘以:

cos (x) \cdot 1 = cos (x)

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

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

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

分式相乘:

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

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

乘以:

1 \cdot sin (x) = sin (x)

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

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

sin (x) 2, 2 cos (x)

的最小公倍数:

2 sin (x) cos (x)

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

最小公倍数 (LCM)

2, 2

的最小公倍数:

2

2, 2

最小公倍数 (LCM)

2

质因数分解:

2

2

2

是质数，因此无法因数分解

= 2

2

质因数分解:

2

2

2

是质数，因此无法因数分解

= 2

将每个因子乘以它在

2

或

2

中出现的最多次数

= 2

数字相乘:

2 = 2

= 2

计算出由出现在

sin (x) 2

或

2 cos (x)

中的因子组成的表达式

= 2 sin (x) cos (x)

根据最小公倍数调整分式

将每个分子乘以其分母转变为最小公倍数所要乘以的同一数值

2 sin (x) cos (x)

对于

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

将分母和分子乘以

cos (x)

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

对于

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

将分母和分子乘以

sin (x)

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

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

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

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

使用三角恒等式改写

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

使用倍角公式:

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

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

使用倍角公式:

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

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

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

使用基本三角恒等式:

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

= cot (2 x)

我们已展示，在两侧可以有相同的形式

\Rightarrow 真

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