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受欢迎的三角函数 >

证明 cos^2(x)+cos^2(x)cot^2(x)=cot^2(x)

解答

证明

cos^{2} (x) + cos^{2} (x) cot^{2} (x) = cot^{2} (x)

解答

真

求解步骤

cos^{2} (x) + cos^{2} (x) cot^{2} (x) = cot^{2} (x)

调整左侧

cos^{2} (x) + cos^{2} (x) cot^{2} (x)

用 sin, cos 表示

cos^{2} (x) + cos^{2} (x) cot^{2} (x)

使用基本三角恒等式:

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

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

化简

cos^{2} (x) + cos^{2} (x) (\frac{cos ( x )}{sin ( x )})^{2} : \frac{cos ^{2} ( x ) sin ^{2} ( x ) + cos ^{4} ( x )}{sin ^{2} ( x )}

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

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

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

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

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

使用指数法则:

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

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

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

分式相乘:

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

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

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

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

使用指数法则:

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

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

= cos^{2 + 2} (x)

数字相加:

2 + 2 = 4

= cos^{4} (x)

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

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

将项转换为分式:

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

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

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

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

分解

\frac{cos ^{4} ( x ) + cos ^{2} ( x ) sin ^{2} ( x )}{sin ^{2} ( x )} : \frac{cos ^{2} ( x ) ( cos ^{2} ( x ) + sin ^{2} ( x ) )}{sin ^{2} ( x )}

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

分解

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

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

使用指数法则:

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

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

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

因式分解出通项

cos^{2} (x)

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

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

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

使用三角恒等式改写

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

使用毕达哥拉斯恒等式:

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

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

化简

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

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

使用三角恒等式改写

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

使用基本三角恒等式:

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

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

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

使用基本三角恒等式:

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

cot (x) cot (x)

化简

cot (x) cot (x) : cot^{2} (x)

cot (x) cot (x)

使用指数法则:

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

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

= cot^{1 + 1} (x)

数字相加:

1 + 1 = 2

= cot^{2} (x)

cot^{2} (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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