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受欢迎的三角函数 >

cos(x)=sin(2x)+cos(3x)

解答

cos (x) = sin (2 x) + cos (3 x)

解答

x = πn, x = \frac{π}{2} + πn, x = \frac{π}{6} + 2 πn, x = \frac{5 π}{6} + 2 πn

+1

度数

x = 0^{\circ} + 18 0^{\circ} n, x = 9 0^{\circ} + 18 0^{\circ} n, x = 3 0^{\circ} + 36 0^{\circ} n, x = 15 0^{\circ} + 36 0^{\circ} n

求解步骤

cos (x) = sin (2 x) + cos (3 x)

两边减去

sin (2 x) + cos (3 x)

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

使用三角恒等式改写

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

化简

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

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

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

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

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

\frac{x + 3 x}{2}

同类项相加:

x + 3 x = 4 x

= \frac{4 x}{2}

数字相除:

\frac{4}{2} = 2

= 2 x

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

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

\frac{x - 3 x}{2}

同类项相加:

x - 3 x = - 2 x

= \frac{- 2 x}{2}

使用分式法则:

\frac{- a}{b} = - \frac{a}{b}

= - \frac{2 x}{2}

数字相除:

\frac{2}{2} = 1

= - x

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

使用负角恒等式:

sin (- x) = - sin (x)

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

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

使用法则

- (- a) = a

= - sin (2 x) + 2 sin (x) sin (2 x)

= - sin (2 x) + 2 sin (x) sin (2 x)

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

分解

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

- sin (2 x) + 2 sin (2 x) sin (x)

因式分解出通项

sin (2 x)

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

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

分别求解每个部分

sin (2 x) = 0 or 2 sin (x) - 1 = 0

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

sin (2 x) = 0

sin (2 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}

2 x = 0 + 2 πn, 2 x = π + 2 πn

2 x = 0 + 2 πn, 2 x = π + 2 πn

解

2 x = 0 + 2 πn : x = πn

2 x = 0 + 2 πn

0 + 2 πn = 2 πn

2 x = 2 πn

两边除以

2

2 x = 2 πn

两边除以

2

\frac{2 x}{2} = \frac{2 πn}{2}

化简

x = πn

x = πn

解

2 x = π + 2 πn : x = \frac{π}{2} + πn

2 x = π + 2 πn

两边除以

2

2 x = π + 2 πn

两边除以

2

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

化简

x = \frac{π}{2} + πn

x = \frac{π}{2} + πn

x = πn, x = \frac{π}{2} + πn

2 sin (x) - 1 = 0 : x = \frac{π}{6} + 2 πn, x = \frac{5 π}{6} + 2 πn

2 sin (x) - 1 = 0

将

1

到右边

2 sin (x) - 1 = 0

两边加上

1

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

化简

2 sin (x) = 1

2 sin (x) = 1

两边除以

2

2 sin (x) = 1

两边除以

2

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

化简

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

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

sin (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}

x = \frac{π}{6} + 2 πn, x = \frac{5 π}{6} + 2 πn

x = \frac{π}{6} + 2 πn, x = \frac{5 π}{6} + 2 πn

合并所有解

x = πn, x = \frac{π}{2} + πn, x = \frac{π}{6} + 2 πn, x = \frac{5 π}{6} + 2 πn

作图

流行的例子

tan(x)sec(x)-2tan(x)=0

tan (x) sec (x) - 2 tan (x) = 0

-9cos^2(θ)+9=17sin(θ)-8

- 9 cos^{2} (θ) + 9 = 17 sin (θ) - 8

4 tan^{2} (θ) - 9 = 0

(sin(90))/(3.37)=(sin(x))/(0.7)

\frac{sin ( 9 0 ^{\circ} )}{3.37} = \frac{sin ( x )}{0.7}

cos(x)=sqrt((1-cos(x))/2)

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