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受欢迎的三角函数 >

cot(θ)=(1+cos^2(θ))/(2sin(θ)cos(θ))

解答

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

解答

θ \in R 无解

求解步骤

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

两边减去

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

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

化简

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

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

将项转换为分式:

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

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

乘开

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

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

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

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

- (1 + cos^{2} (θ))

打开括号

= - (1) - (cos^{2} (θ))

使用加减运算法则

+ (- a) = - a

= - 1 - cos^{2} (θ)

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

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

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

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

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

使用三角恒等式改写

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

使用基本三角恒等式:

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

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

化简

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

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

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

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

分式相乘:

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

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

约分:

sin (θ)

= cos (θ) \cdot 2 cos (θ)

使用指数法则:

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

cos (θ) cos (θ) = cos^{1 + 1} (θ)

= 2 cos^{1 + 1} (θ)

数字相加:

1 + 1 = 2

= 2 cos^{2} (θ)

= - 1 - cos^{2} (θ) + 2 cos^{2} (θ)

同类项相加:

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

= - 1 + cos^{2} (θ)

= - 1 + cos^{2} (θ)

- 1 + cos^{2} (θ) = 0

用替代法求解

- 1 + cos^{2} (θ) = 0

令

: cos (θ) = u

- 1 + u^{2} = 0

- 1 + u^{2} = 0 : u = 1, u = - 1

- 1 + u^{2} = 0

将

1

到右边

- 1 + u^{2} = 0

两边加上

1

- 1 + u^{2} + 1 = 0 + 1

化简

u^{2} = 1

u^{2} = 1

对于

x^{2} = f (a)

解为

x = f (a), - f (a)

u = 1, u = - 1

1 = 1

1

使用法则

1 = 1

= 1

- 1 = - 1

- 1

使用法则

1 = 1

= - 1

u = 1, u = - 1

u = cos (θ)

代回

cos (θ) = 1, cos (θ) = - 1

cos (θ) = 1, cos (θ) = - 1

cos (θ) = 1 : θ = 2 πn

cos (θ) = 1

cos (θ) = 1

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 = 0 + 2 πn

θ = 0 + 2 πn

解

θ = 0 + 2 πn : θ = 2 πn

θ = 0 + 2 πn

0 + 2 πn = 2 πn

θ = 2 πn

θ = 2 πn

cos (θ) = - 1 : θ = π + 2 πn

cos (θ) = - 1

cos (θ) = - 1

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 = π + 2 πn

θ = π + 2 πn

合并所有解

θ = 2 πn, θ = π + 2 πn

因为方程对以下值无定义:

2 πn, π + 2 πn

θ \in R 无解

作图

流行的例子

sec(3x)-csc(30)=0,(x+35)/5

4cos(2θ)-10cos(θ)+14=7,0<= θ<360

-2sin(θ)=sqrt(2)

tan(2x-pi/6)= 1/(3^{1/2)}