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受欢迎的三角函数 >

cos(4x)-cos(2x)=0

解答

cos (4 x) - cos (2 x) = 0

解答

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

+1

度数

x = 0^{\circ} + 12 0^{\circ} n, x = 6 0^{\circ} + 12 0^{\circ} n

求解步骤

cos (4 x) - cos (2 x) = 0

使用三角恒等式改写

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

化简

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

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

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

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

同类项相加:

4 x + 2 x = 6 x

= \frac{6 x}{2}

数字相除:

\frac{6}{2} = 3

= 3 x

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

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

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

同类项相加:

4 x - 2 x = 2 x

= \frac{2 x}{2}

数字相除:

\frac{2}{2} = 1

= x

= - 2 sin (3 x) sin (x)

= - 2 sin (3 x) sin (x)

- 2 sin (3 x) sin (x) = 0

分别求解每个部分

sin (3 x) = 0 or sin (x) = 0

sin (3 x) = 0 : x = \frac{2 πn}{3}, x = \frac{π}{3} + \frac{2 πn}{3}

sin (3 x) = 0

sin (3 x) = 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}

3 x = 0 + 2 πn, 3 x = π + 2 πn

3 x = 0 + 2 πn, 3 x = π + 2 πn

解

3 x = 0 + 2 πn : x = \frac{2 πn}{3}

3 x = 0 + 2 πn

0 + 2 πn = 2 πn

3 x = 2 πn

两边除以

3

3 x = 2 πn

两边除以

3

\frac{3 x}{3} = \frac{2 πn}{3}

化简

x = \frac{2 πn}{3}

x = \frac{2 πn}{3}

解

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

3 x = π + 2 πn

两边除以

3

3 x = π + 2 πn

两边除以

3

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

化简

x = \frac{π}{3} + \frac{2 πn}{3}

x = \frac{π}{3} + \frac{2 πn}{3}

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

sin (x) = 0 : x = 2 πn, x = π + 2 πn

sin (x) = 0

sin (x) = 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}

x = 0 + 2 πn, x = π + 2 πn

x = 0 + 2 πn, x = π + 2 πn

解

x = 0 + 2 πn : x = 2 πn

x = 0 + 2 πn

0 + 2 πn = 2 πn

x = 2 πn

x = 2 πn, x = π + 2 πn

合并所有解

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

合并重叠的区间

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

作图

流行的例子

2sin(x)cos(x)-cos(x)=0

2 sin (x) cos (x) - cos (x) = 0

2cos(θ)=-sqrt(2)

2 cos (θ) = - 2

tan(x/3)=sqrt(3)

tan (\frac{x}{3}) = 3

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

cos (2 x) + 1 = 0