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受欢迎的三角函数 >

证明-(-1+sin^2(θ))/(sin^2(θ))=cot^2(θ)

解答

证明

- \frac{- 1 + sin ^{2} ( θ )}{sin ^{2} ( θ )} = cot^{2} (θ)

解答

真

求解步骤

- \frac{- 1 + sin ^{2} ( θ )}{sin ^{2} ( θ )} = cot^{2} (θ)

调整右侧

cot^{2} (θ)

用 sin, cos 表示

cot^{2} (θ)

使用基本三角恒等式:

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

= (\frac{cos ( θ )}{sin ( θ )})^{2}

化简

(\frac{cos ( θ )}{sin ( θ )})^{2} : \frac{cos ^{2} ( θ )}{sin ^{2} ( θ )}

(\frac{cos ( θ )}{sin ( θ )})^{2}

使用指数法则:

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

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

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

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

使用三角恒等式改写

\frac{cos ^{2} ( θ )}{sin ^{2} ( θ )}

使用倍角公式:

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

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

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

使用毕达哥拉斯恒等式:

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

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

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

使用倍角公式:

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

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

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

cos^{2} (θ) - sin^{2} (θ) + 1 - cos^{2} (θ)

对同类项分组

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

同类项相加:

cos^{2} (θ) - cos^{2} (θ) = 0

= - sin^{2} (θ) + 1

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

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

分解

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

\frac{1 - sin ^{2} ( θ )}{1 - cos ^{2} ( θ )}

分解

1 - cos^{2} (θ) : - (cos (θ) + 1) (cos (θ) - 1)

1 - cos^{2} (θ)

因式分解出通项

- 1

= - (cos^{2} (θ) - 1)

分解

cos^{2} (θ) - 1 : (cos (θ) + 1) (cos (θ) - 1)

cos^{2} (θ) - 1

将

1

改写为

1^{2}

= cos^{2} (θ) - 1^{2}

使用平方差公式:

x^{2} - y^{2} = (x + y) (x - y)

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

= (cos (θ) + 1) (cos (θ) - 1)

= - (cos (θ) + 1) (cos (θ) - 1)

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

分解

1 - sin^{2} (θ) : - (sin (θ) + 1) (sin (θ) - 1)

1 - sin^{2} (θ)

因式分解出通项

- 1

= - (sin^{2} (θ) - 1)

分解

sin^{2} (θ) - 1 : (sin (θ) + 1) (sin (θ) - 1)

sin^{2} (θ) - 1

将

1

改写为

1^{2}

= sin^{2} (θ) - 1^{2}

使用平方差公式:

x^{2} - y^{2} = (x + y) (x - y)

sin^{2} (θ) - 1^{2} = (sin (θ) + 1) (sin (θ) - 1)

= (sin (θ) + 1) (sin (θ) - 1)

= - (sin (θ) + 1) (sin (θ) - 1)

= \frac{- ( sin ( θ ) + 1 ) ( sin ( θ ) - 1 )}{- ( cos ( θ ) + 1 ) ( cos ( θ ) - 1 )}

= \frac{- ( - 1 + sin ( θ ) ) ( 1 + sin ( θ ) )}{- ( - 1 + cos ( θ ) ) ( 1 + cos ( θ ) )}

乘开

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

\frac{- ( - 1 + sin ( θ ) ) ( 1 + sin ( θ ) )}{- ( - 1 + cos ( θ ) ) ( 1 + cos ( θ ) )}

使用分式法则:

\frac{- a}{- b} = \frac{a}{b}

= \frac{( - 1 + sin ( θ ) ) ( 1 + sin ( θ ) )}{( - 1 + cos ( θ ) ) ( 1 + cos ( θ ) )}

乘开

(- 1 + cos (θ)) (1 + cos (θ)) : cos^{2} (θ) - 1

(- 1 + cos (θ)) (1 + cos (θ))

使用平方差公式:

(a - b) (a + b) = a^{2} - b^{2}

a = cos (θ), b = 1

= cos^{2} (θ) - 1^{2}

使用法则

1^{a} = 1

1^{2} = 1

= cos^{2} (θ) - 1

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

乘开

(- 1 + sin (θ)) (1 + sin (θ)) : sin^{2} (θ) - 1

(- 1 + sin (θ)) (1 + sin (θ))

使用平方差公式:

(a - b) (a + b) = a^{2} - b^{2}

a = sin (θ), b = 1

= sin^{2} (θ) - 1^{2}

使用法则

1^{a} = 1

1^{2} = 1

= sin^{2} (θ) - 1

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

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

使用三角恒等式改写

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

使用毕达哥拉斯恒等式:

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

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

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

化简

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

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

我们已展示，在两侧可以有相同的形式

\Rightarrow 真

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