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受欢迎的三角函数 >

cos(8x)-cos(2x)=sin(5x)

解答

cos (8 x) - cos (2 x) = sin (5 x)

解答

x = \frac{2 πn}{5}, x = \frac{π}{5} + \frac{2 πn}{5}, x = \frac{7 π}{18} + \frac{2 πn}{3}, x = \frac{11 π}{18} + \frac{2 πn}{3}

+1

度数

x = 0^{\circ} + 7 2^{\circ} n, x = 3 6^{\circ} + 7 2^{\circ} n, x = 7 0^{\circ} + 12 0^{\circ} n, x = 11 0^{\circ} + 12 0^{\circ} n

求解步骤

cos (8 x) - cos (2 x) = sin (5 x)

两边减去

sin (5 x)

cos (8 x) - cos (2 x) - sin (5 x) = 0

使用三角恒等式改写

- cos (2 x) + cos (8 x) - sin (5 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})

= - sin (5 x) - 2 sin (\frac{8 x + 2 x}{2}) sin (\frac{8 x - 2 x}{2})

2 sin (\frac{8 x + 2 x}{2}) sin (\frac{8 x - 2 x}{2}) = 2 sin (5 x) sin (3 x)

2 sin (\frac{8 x + 2 x}{2}) sin (\frac{8 x - 2 x}{2})

\frac{8 x + 2 x}{2} = 5 x

\frac{8 x + 2 x}{2}

同类项相加:

8 x + 2 x = 10 x

= \frac{10 x}{2}

数字相除:

\frac{10}{2} = 5

= 5 x

= 2 sin (5 x) sin (\frac{8 x - 2 x}{2})

\frac{8 x - 2 x}{2} = 3 x

\frac{8 x - 2 x}{2}

同类项相加:

8 x - 2 x = 6 x

= \frac{6 x}{2}

数字相除:

\frac{6}{2} = 3

= 3 x

= 2 sin (5 x) sin (3 x)

= - sin (5 x) - 2 sin (5 x) sin (3 x)

- sin (5 x) - 2 sin (3 x) sin (5 x) = 0

分解

- sin (5 x) - 2 sin (3 x) sin (5 x) : - sin (5 x) (2 sin (3 x) + 1)

- sin (5 x) - 2 sin (3 x) sin (5 x)

因式分解出通项

- sin (5 x)

= - sin (5 x) (1 + 2 sin (3 x))

- sin (5 x) (2 sin (3 x) + 1) = 0

分别求解每个部分

sin (5 x) = 0 or 2 sin (3 x) + 1 = 0

sin (5 x) = 0 : x = \frac{2 πn}{5}, x = \frac{π}{5} + \frac{2 πn}{5}

sin (5 x) = 0

sin (5 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}

5 x = 0 + 2 πn, 5 x = π + 2 πn

5 x = 0 + 2 πn, 5 x = π + 2 πn

解

5 x = 0 + 2 πn : x = \frac{2 πn}{5}

5 x = 0 + 2 πn

0 + 2 πn = 2 πn

5 x = 2 πn

两边除以

5

5 x = 2 πn

两边除以

5

\frac{5 x}{5} = \frac{2 πn}{5}

化简

x = \frac{2 πn}{5}

x = \frac{2 πn}{5}

解

5 x = π + 2 πn : x = \frac{π}{5} + \frac{2 πn}{5}

5 x = π + 2 πn

两边除以

5

5 x = π + 2 πn

两边除以

5

\frac{5 x}{5} = \frac{π}{5} + \frac{2 πn}{5}

化简

x = \frac{π}{5} + \frac{2 πn}{5}

x = \frac{π}{5} + \frac{2 πn}{5}

x = \frac{2 πn}{5}, x = \frac{π}{5} + \frac{2 πn}{5}

2 sin (3 x) + 1 = 0 : x = \frac{7 π}{18} + \frac{2 πn}{3}, x = \frac{11 π}{18} + \frac{2 πn}{3}

2 sin (3 x) + 1 = 0

将

1

到右边

2 sin (3 x) + 1 = 0

两边减去

1

2 sin (3 x) + 1 - 1 = 0 - 1

化简

2 sin (3 x) = - 1

2 sin (3 x) = - 1

两边除以

2

2 sin (3 x) = - 1

两边除以

2

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

化简

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

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

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

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 = \frac{7 π}{6} + 2 πn, 3 x = \frac{11 π}{6} + 2 πn

3 x = \frac{7 π}{6} + 2 πn, 3 x = \frac{11 π}{6} + 2 πn

解

3 x = \frac{7 π}{6} + 2 πn : x = \frac{7 π}{18} + \frac{2 πn}{3}

3 x = \frac{7 π}{6} + 2 πn

两边除以

3

3 x = \frac{7 π}{6} + 2 πn

两边除以

3

\frac{3 x}{3} = \frac{\frac{7 π}{6}}{3} + \frac{2 πn}{3}

化简

\frac{3 x}{3} = \frac{\frac{7 π}{6}}{3} + \frac{2 πn}{3}

化简

\frac{3 x}{3} : x

\frac{3 x}{3}

数字相除:

\frac{3}{3} = 1

= x

化简

\frac{\frac{7 π}{6}}{3} + \frac{2 πn}{3} : \frac{7 π}{18} + \frac{2 πn}{3}

\frac{\frac{7 π}{6}}{3} + \frac{2 πn}{3}

\frac{\frac{7 π}{6}}{3} = \frac{7 π}{18}

\frac{\frac{7 π}{6}}{3}

使用分式法则:

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

= \frac{7 π}{6 \cdot 3}

数字相乘:

6 \cdot 3 = 18

= \frac{7 π}{18}

= \frac{7 π}{18} + \frac{2 πn}{3}

x = \frac{7 π}{18} + \frac{2 πn}{3}

x = \frac{7 π}{18} + \frac{2 πn}{3}

x = \frac{7 π}{18} + \frac{2 πn}{3}

解

3 x = \frac{11 π}{6} + 2 πn : x = \frac{11 π}{18} + \frac{2 πn}{3}

3 x = \frac{11 π}{6} + 2 πn

两边除以

3

3 x = \frac{11 π}{6} + 2 πn

两边除以

3

\frac{3 x}{3} = \frac{\frac{11 π}{6}}{3} + \frac{2 πn}{3}

化简

\frac{3 x}{3} = \frac{\frac{11 π}{6}}{3} + \frac{2 πn}{3}

化简

\frac{3 x}{3} : x

\frac{3 x}{3}

数字相除:

\frac{3}{3} = 1

= x

化简

\frac{\frac{11 π}{6}}{3} + \frac{2 πn}{3} : \frac{11 π}{18} + \frac{2 πn}{3}

\frac{\frac{11 π}{6}}{3} + \frac{2 πn}{3}

\frac{\frac{11 π}{6}}{3} = \frac{11 π}{18}

\frac{\frac{11 π}{6}}{3}

使用分式法则:

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

= \frac{11 π}{6 \cdot 3}

数字相乘:

6 \cdot 3 = 18

= \frac{11 π}{18}

= \frac{11 π}{18} + \frac{2 πn}{3}

x = \frac{11 π}{18} + \frac{2 πn}{3}

x = \frac{11 π}{18} + \frac{2 πn}{3}

x = \frac{11 π}{18} + \frac{2 πn}{3}

x = \frac{7 π}{18} + \frac{2 πn}{3}, x = \frac{11 π}{18} + \frac{2 πn}{3}

合并所有解

x = \frac{2 πn}{5}, x = \frac{π}{5} + \frac{2 πn}{5}, x = \frac{7 π}{18} + \frac{2 πn}{3}, x = \frac{11 π}{18} + \frac{2 πn}{3}

作图

流行的例子

tan (x) = - \frac{1}{5}

sqrt(2)+2cos(x)=0

2 + 2 cos (x) = 0

2sin(θ)sec(θ)=4sin(θ)

2 sin (θ) sec (θ) = 4 sin (θ)

sec(x-pi/7)cos(x-pi/7)=sin(x-pi/7)

sec (x - \frac{π}{7}) cos (x - \frac{π}{7}) = sin (x - \frac{π}{7})

cos (x) = \frac{17}{28}