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受欢迎的三角函数 >

cos(7x)+cos(3x)=0

解答

cos (7 x) + cos (3 x) = 0

解答

x = \frac{π}{4} + πn, x = \frac{3 π}{4} + πn, x = \frac{π}{10} + \frac{2 πn}{5}, x = \frac{3 π}{10} + \frac{2 πn}{5}

+1

度数

x = 4 5^{\circ} + 18 0^{\circ} n, x = 13 5^{\circ} + 18 0^{\circ} n, x = 1 8^{\circ} + 7 2^{\circ} n, x = 5 4^{\circ} + 7 2^{\circ} n

求解步骤

cos (7 x) + cos (3 x) = 0

使用三角恒等式改写

cos (3 x) + cos (7 x)

使用和差化积恒等式:

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

= 2 cos (\frac{3 x + 7 x}{2}) cos (\frac{3 x - 7 x}{2})

化简

2 cos (\frac{3 x + 7 x}{2}) cos (\frac{3 x - 7 x}{2}) : 2 cos (2 x) cos (5 x)

2 cos (\frac{3 x + 7 x}{2}) cos (\frac{3 x - 7 x}{2})

\frac{3 x + 7 x}{2} = 5 x

\frac{3 x + 7 x}{2}

同类项相加:

3 x + 7 x = 10 x

= \frac{10 x}{2}

数字相除:

\frac{10}{2} = 5

= 5 x

= 2 cos (5 x) cos (\frac{3 x - 7 x}{2})

\frac{3 x - 7 x}{2} = - 2 x

\frac{3 x - 7 x}{2}

同类项相加:

3 x - 7 x = - 4 x

= \frac{- 4 x}{2}

使用分式法则:

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

= - \frac{4 x}{2}

数字相除:

\frac{4}{2} = 2

= - 2 x

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

使用负角恒等式:

cos (- x) = cos (x)

= 2 cos (2 x) cos (5 x)

= 2 cos (2 x) cos (5 x)

2 cos (2 x) cos (5 x) = 0

分别求解每个部分

cos (2 x) = 0 or cos (5 x) = 0

cos (2 x) = 0 : x = \frac{π}{4} + πn, x = \frac{3 π}{4} + πn

cos (2 x) = 0

cos (2 x) = 0

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}

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

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

解

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

2 x = \frac{π}{2} + 2 πn

两边除以

2

2 x = \frac{π}{2} + 2 πn

两边除以

2

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

化简

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

化简

\frac{2 x}{2} : x

\frac{2 x}{2}

数字相除:

\frac{2}{2} = 1

= x

化简

\frac{\frac{π}{2}}{2} + \frac{2 πn}{2} : \frac{π}{4} + πn

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

\frac{\frac{π}{2}}{2} = \frac{π}{4}

\frac{\frac{π}{2}}{2}

使用分式法则:

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

= \frac{π}{2 \cdot 2}

数字相乘:

2 \cdot 2 = 4

= \frac{π}{4}

\frac{2 πn}{2} = πn

\frac{2 πn}{2}

数字相除:

\frac{2}{2} = 1

= πn

= \frac{π}{4} + πn

x = \frac{π}{4} + πn

x = \frac{π}{4} + πn

x = \frac{π}{4} + πn

解

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

2 x = \frac{3 π}{2} + 2 πn

两边除以

2

2 x = \frac{3 π}{2} + 2 πn

两边除以

2

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

化简

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

化简

\frac{2 x}{2} : x

\frac{2 x}{2}

数字相除:

\frac{2}{2} = 1

= x

化简

\frac{\frac{3 π}{2}}{2} + \frac{2 πn}{2} : \frac{3 π}{4} + πn

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

\frac{\frac{3 π}{2}}{2} = \frac{3 π}{4}

\frac{\frac{3 π}{2}}{2}

使用分式法则:

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

= \frac{3 π}{2 \cdot 2}

数字相乘:

2 \cdot 2 = 4

= \frac{3 π}{4}

\frac{2 πn}{2} = πn

\frac{2 πn}{2}

数字相除:

\frac{2}{2} = 1

= πn

= \frac{3 π}{4} + πn

x = \frac{3 π}{4} + πn

x = \frac{3 π}{4} + πn

x = \frac{3 π}{4} + πn

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

cos (5 x) = 0 : x = \frac{π}{10} + \frac{2 πn}{5}, x = \frac{3 π}{10} + \frac{2 πn}{5}

cos (5 x) = 0

cos (5 x) = 0

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}

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

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

解

5 x = \frac{π}{2} + 2 πn : x = \frac{π}{10} + \frac{2 πn}{5}

5 x = \frac{π}{2} + 2 πn

两边除以

5

5 x = \frac{π}{2} + 2 πn

两边除以

5

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

化简

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

化简

\frac{5 x}{5} : x

\frac{5 x}{5}

数字相除:

\frac{5}{5} = 1

= x

化简

\frac{\frac{π}{2}}{5} + \frac{2 πn}{5} : \frac{π}{10} + \frac{2 πn}{5}

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

\frac{\frac{π}{2}}{5} = \frac{π}{10}

\frac{\frac{π}{2}}{5}

使用分式法则:

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

= \frac{π}{2 \cdot 5}

数字相乘:

2 \cdot 5 = 10

= \frac{π}{10}

= \frac{π}{10} + \frac{2 πn}{5}

x = \frac{π}{10} + \frac{2 πn}{5}

x = \frac{π}{10} + \frac{2 πn}{5}

x = \frac{π}{10} + \frac{2 πn}{5}

解

5 x = \frac{3 π}{2} + 2 πn : x = \frac{3 π}{10} + \frac{2 πn}{5}

5 x = \frac{3 π}{2} + 2 πn

两边除以

5

5 x = \frac{3 π}{2} + 2 πn

两边除以

5

\frac{5 x}{5} = \frac{\frac{3 π}{2}}{5} + \frac{2 πn}{5}

化简

\frac{5 x}{5} = \frac{\frac{3 π}{2}}{5} + \frac{2 πn}{5}

化简

\frac{5 x}{5} : x

\frac{5 x}{5}

数字相除:

\frac{5}{5} = 1

= x

化简

\frac{\frac{3 π}{2}}{5} + \frac{2 πn}{5} : \frac{3 π}{10} + \frac{2 πn}{5}

\frac{\frac{3 π}{2}}{5} + \frac{2 πn}{5}

\frac{\frac{3 π}{2}}{5} = \frac{3 π}{10}

\frac{\frac{3 π}{2}}{5}

使用分式法则:

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

= \frac{3 π}{2 \cdot 5}

数字相乘:

2 \cdot 5 = 10

= \frac{3 π}{10}

= \frac{3 π}{10} + \frac{2 πn}{5}

x = \frac{3 π}{10} + \frac{2 πn}{5}

x = \frac{3 π}{10} + \frac{2 πn}{5}

x = \frac{3 π}{10} + \frac{2 πn}{5}

x = \frac{π}{10} + \frac{2 πn}{5}, x = \frac{3 π}{10} + \frac{2 πn}{5}

合并所有解

x = \frac{π}{4} + πn, x = \frac{3 π}{4} + πn, x = \frac{π}{10} + \frac{2 πn}{5}, x = \frac{3 π}{10} + \frac{2 πn}{5}

作图

流行的例子

5sin^2(x)+2sin(x)=0

5 sin^{2} (x) + 2 sin (x) = 0

cos(2x)-11cos(x)+6=0

cos (2 x) - 11 cos (x) + 6 = 0

sin (x) = - 0.45

sin (x) = - 0.59

sin (x) = - 0.55