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受欢迎的三角函数 >

sec^2(θ)-2=tan^2(θ)

解答

sec^{2} (θ) - 2 = tan^{2} (θ)

解答

θ \in R 无解

求解步骤

sec^{2} (θ) - 2 = tan^{2} (θ)

两边减去

tan^{2} (θ)

sec^{2} (θ) - 2 - tan^{2} (θ) = 0

用 sin, cos 表示

- 2 + sec^{2} (θ) - tan^{2} (θ)

使用基本三角恒等式:

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

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

使用基本三角恒等式:

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

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

化简

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

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

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

(\frac{1}{cos ( θ )})^{2}

使用指数法则:

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

= \frac{1 ^{2}}{cos ^{2} ( θ )}

使用法则

1^{a} = 1

1^{2} = 1

= \frac{1}{cos ^{2} ( θ )}

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

(\frac{sin ( θ )}{cos ( θ )})^{2}

使用指数法则:

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

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

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

合并分式

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

使用法则

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

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

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

将项转换为分式:

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

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

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

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

\frac{1 - sin ^{2} ( θ ) - 2 cos ^{2} ( θ )}{cos ^{2} ( θ )} = 0

\frac{f ( x )}{g ( x )} = 0 \Rightarrow f (x) = 0

1 - sin^{2} (θ) - 2 cos^{2} (θ) = 0

使用三角恒等式改写

1 - sin^{2} (θ) - 2 cos^{2} (θ)

使用毕达哥拉斯恒等式:

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

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

= - 2 cos^{2} (θ) + cos^{2} (θ)

化简

= - cos^{2} (θ)

- cos^{2} (θ) = 0

两边除以

- 1

- cos^{2} (θ) = 0

两边除以

- 1

- cos^{2} (θ) = 0

两边除以

- 1

\frac{- cos ^{2} ( θ )}{- 1} = \frac{0}{- 1}

化简

cos^{2} (θ) = 0

cos^{2} (θ) = 0

使用法则

x^{n} = 0 \Rightarrow x = 0

cos (θ) = 0

cos (θ) = 0

的通解

cos (x)

周期表（周期为

2 πn

）：

x 0 \frac{π}{6} \frac{π}{4} \frac{π}{3} \frac{π}{2} \frac{2 π}{3} \frac{3 π}{4} \frac{5 π}{6} cos (x) 1 \frac{3}{2} \frac{2}{2} \frac{1}{2} 0 - \frac{1}{2} - \frac{2}{2} - \frac{3}{2} x π \frac{7 π}{6} \frac{5 π}{4} \frac{4 π}{3} \frac{3 π}{2} \frac{5 π}{3} \frac{7 π}{4} \frac{11 π}{6} cos (x) - 1 - \frac{3}{2} - \frac{2}{2} - \frac{1}{2} 0 \frac{1}{2} \frac{2}{2} \frac{3}{2}

θ = \frac{π}{2} + 2 πn, θ = \frac{3 π}{2} + 2 πn

θ = \frac{π}{2} + 2 πn, θ = \frac{3 π}{2} + 2 πn

因为方程对以下值无定义:

\frac{π}{2} + 2 πn, \frac{3 π}{2} + 2 πn

θ \in R 无解

作图

流行的例子

cos(x)=sin(x)+1

cos (x) = sin (x) + 1

2sin^2(θ)=3(1-cos(-θ))

2 sin^{2} (θ) = 3 (1 - cos (- θ))

sec(x)-sqrt(2)=0

sec (x) - 2 = 0

2sin^4(x)+sin^2(x)-1=0

2 sin^{4} (x) + sin^{2} (x) - 1 = 0

tan(x/9)+sqrt(3)=0

tan (\frac{x}{9}) + 3 = 0