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受欢迎的三角函数 >

cos(x)=cos(x/2)

解答

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

解答

x = \frac{8 πn}{3}, x = \frac{4 π}{3} + \frac{8 πn}{3}

+1

度数

x = 0^{\circ} + 48 0^{\circ} n, x = 24 0^{\circ} + 48 0^{\circ} n

求解步骤

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

两边减去

cos (\frac{x}{2})

cos (x) - cos (\frac{x}{2}) = 0

使用三角恒等式改写

- cos (\frac{x}{2}) + cos (x)

使用和差化积恒等式:

cos (s) - cos (t) = - 2 sin (\frac{s + t}{2}) sin (\frac{s - t}{2})

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

化简

- 2 sin (\frac{x + \frac{x}{2}}{2}) sin (\frac{x - \frac{x}{2}}{2}) : - 2 sin (\frac{3 x}{4}) sin (\frac{x}{4})

- 2 sin (\frac{x + \frac{x}{2}}{2}) sin (\frac{x - \frac{x}{2}}{2})

\frac{x + \frac{x}{2}}{2} = \frac{3 x}{4}

\frac{x + \frac{x}{2}}{2}

化简

x + \frac{x}{2} : \frac{3 x}{2}

x + \frac{x}{2}

将项转换为分式:

x = \frac{x 2}{2}

= \frac{x \cdot 2}{2} + \frac{x}{2}

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{x \cdot 2 + x}{2}

同类项相加:

2 x + x = 3 x

= \frac{3 x}{2}

= \frac{\frac{3 x}{2}}{2}

使用分式法则:

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

= \frac{3 x}{2 \cdot 2}

数字相乘:

2 \cdot 2 = 4

= \frac{3 x}{4}

= - 2 sin (\frac{3 x}{4}) sin (\frac{x - \frac{x}{2}}{2})

\frac{x - \frac{x}{2}}{2} = \frac{x}{4}

\frac{x - \frac{x}{2}}{2}

化简

x - \frac{x}{2} : \frac{x}{2}

x - \frac{x}{2}

将项转换为分式:

x = \frac{x 2}{2}

= \frac{x \cdot 2}{2} - \frac{x}{2}

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{x \cdot 2 - x}{2}

同类项相加:

2 x - x = x

= \frac{x}{2}

= \frac{\frac{x}{2}}{2}

使用分式法则:

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

= \frac{x}{2 \cdot 2}

数字相乘:

2 \cdot 2 = 4

= \frac{x}{4}

= - 2 sin (\frac{3 x}{4}) sin (\frac{x}{4})

= - 2 sin (\frac{3 x}{4}) sin (\frac{x}{4})

- 2 sin (\frac{3 x}{4}) sin (\frac{x}{4}) = 0

分别求解每个部分

sin (\frac{3 x}{4}) = 0 or sin (\frac{x}{4}) = 0

sin (\frac{3 x}{4}) = 0 : x = \frac{8 πn}{3}, x = \frac{4 π}{3} + \frac{8 πn}{3}

sin (\frac{3 x}{4}) = 0

sin (\frac{3 x}{4}) = 0

的通解

sin (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} sin (x) 0 \frac{1}{2} \frac{2}{2} \frac{3}{2} 1 \frac{3}{2} \frac{2}{2} \frac{1}{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} sin (x) 0 - \frac{1}{2} - \frac{2}{2} - \frac{3}{2} - 1 - \frac{3}{2} - \frac{2}{2} - \frac{1}{2}

\frac{3 x}{4} = 0 + 2 πn, \frac{3 x}{4} = π + 2 πn

\frac{3 x}{4} = 0 + 2 πn, \frac{3 x}{4} = π + 2 πn

解

\frac{3 x}{4} = 0 + 2 πn : x = \frac{8 πn}{3}

\frac{3 x}{4} = 0 + 2 πn

0 + 2 πn = 2 πn

\frac{3 x}{4} = 2 πn

在两边乘以

4

\frac{3 x}{4} = 2 πn

在两边乘以

4

\frac{4 \cdot 3 x}{4} = 4 \cdot 2 πn

化简

3 x = 8 πn

3 x = 8 πn

两边除以

3

3 x = 8 πn

两边除以

3

\frac{3 x}{3} = \frac{8 πn}{3}

化简

x = \frac{8 πn}{3}

x = \frac{8 πn}{3}

解

\frac{3 x}{4} = π + 2 πn : x = \frac{4 π}{3} + \frac{8 πn}{3}

\frac{3 x}{4} = π + 2 πn

在两边乘以

4

\frac{3 x}{4} = π + 2 πn

在两边乘以

4

\frac{4 \cdot 3 x}{4} = 4 π + 4 \cdot 2 πn

化简

3 x = 4 π + 8 πn

3 x = 4 π + 8 πn

两边除以

3

3 x = 4 π + 8 πn

两边除以

3

\frac{3 x}{3} = \frac{4 π}{3} + \frac{8 πn}{3}

化简

x = \frac{4 π}{3} + \frac{8 πn}{3}

x = \frac{4 π}{3} + \frac{8 πn}{3}

x = \frac{8 πn}{3}, x = \frac{4 π}{3} + \frac{8 πn}{3}

sin (\frac{x}{4}) = 0 : x = 8 πn, x = 4 π + 8 πn

sin (\frac{x}{4}) = 0

sin (\frac{x}{4}) = 0

的通解

sin (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} sin (x) 0 \frac{1}{2} \frac{2}{2} \frac{3}{2} 1 \frac{3}{2} \frac{2}{2} \frac{1}{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} sin (x) 0 - \frac{1}{2} - \frac{2}{2} - \frac{3}{2} - 1 - \frac{3}{2} - \frac{2}{2} - \frac{1}{2}

\frac{x}{4} = 0 + 2 πn, \frac{x}{4} = π + 2 πn

\frac{x}{4} = 0 + 2 πn, \frac{x}{4} = π + 2 πn

解

\frac{x}{4} = 0 + 2 πn : x = 8 πn

\frac{x}{4} = 0 + 2 πn

0 + 2 πn = 2 πn

\frac{x}{4} = 2 πn

在两边乘以

4

\frac{x}{4} = 2 πn

在两边乘以

4

\frac{4 x}{4} = 4 \cdot 2 πn

化简

x = 8 πn

x = 8 πn

解

\frac{x}{4} = π + 2 πn : x = 4 π + 8 πn

\frac{x}{4} = π + 2 πn

在两边乘以

4

\frac{x}{4} = π + 2 πn

在两边乘以

4

\frac{4 x}{4} = 4 π + 4 \cdot 2 πn

化简

x = 4 π + 8 πn

x = 4 π + 8 πn

x = 8 πn, x = 4 π + 8 πn

合并所有解

x = \frac{8 πn}{3}, x = \frac{4 π}{3} + \frac{8 πn}{3}, x = 8 πn, x = 4 π + 8 πn

合并重叠的区间

x = \frac{8 πn}{3}, x = \frac{4 π}{3} + \frac{8 πn}{3}

作图

流行的例子

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3 tan^{2} (x) - 3 tan (x) = 0

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sqrt(2)cos(x)+sqrt(2)sin(x)=1

2 cos (x) + 2 sin (x) = 1

sin(θ)= 2/(sqrt(13)),cos(θ)= 3/(sqrt(13))

sin (θ) = \frac{2}{13}, cos (θ) = \frac{3}{13}

6sin^2(x)+4sin(x)-1=0

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