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受欢迎的三角函数 >

5cos^2(x)+3cos(x)-2=0

解答

5 cos^{2} (x) + 3 cos (x) - 2 = 0

解答

x = 1.15927 \dots + 2 πn, x = 2 π - 1.15927 \dots + 2 πn, x = π + 2 πn

+1

度数

x = 66.42182 \dots^{\circ} + 36 0^{\circ} n, x = 293.57817 \dots^{\circ} + 36 0^{\circ} n, x = 18 0^{\circ} + 36 0^{\circ} n

求解步骤

5 cos^{2} (x) + 3 cos (x) - 2 = 0

用替代法求解

5 cos^{2} (x) + 3 cos (x) - 2 = 0

令

: cos (x) = u

5 u^{2} + 3 u - 2 = 0

5 u^{2} + 3 u - 2 = 0 : u = \frac{2}{5}, u = - 1

5 u^{2} + 3 u - 2 = 0

使用求根公式求解

5 u^{2} + 3 u - 2 = 0

二次方程求根公式:

若

a = 5, b = 3, c = - 2

u_{1, 2} = \frac{- 3 \pm 3 ^{2} - 4 \cdot 5 ( - 2 )}{2 \cdot 5}

u_{1, 2} = \frac{- 3 \pm 3 ^{2} - 4 \cdot 5 ( - 2 )}{2 \cdot 5}

3^{2} - 4 \cdot 5 (- 2) = 7

3^{2} - 4 \cdot 5 (- 2)

使用法则

- (- a) = a

= 3^{2} + 4 \cdot 5 \cdot 2

数字相乘:

4 \cdot 5 \cdot 2 = 40

= 3^{2} + 40

3^{2} = 9

= 9 + 40

数字相加:

9 + 40 = 49

= 49

因式分解数字:

49 = 7^{2}

= 7^{2}

使用根式运算法则:

n a^{n} = a

7^{2} = 7

= 7

u_{1, 2} = \frac{- 3 \pm 7}{2 \cdot 5}

将解分隔开

u_{1} = \frac{- 3 + 7}{2 \cdot 5}, u_{2} = \frac{- 3 - 7}{2 \cdot 5}

u = \frac{- 3 + 7}{2 \cdot 5} : \frac{2}{5}

\frac{- 3 + 7}{2 \cdot 5}

数字相加/相减:

- 3 + 7 = 4

= \frac{4}{2 \cdot 5}

数字相乘:

2 \cdot 5 = 10

= \frac{4}{10}

约分:

2

= \frac{2}{5}

u = \frac{- 3 - 7}{2 \cdot 5} : - 1

\frac{- 3 - 7}{2 \cdot 5}

数字相减:

- 3 - 7 = - 10

= \frac{- 10}{2 \cdot 5}

数字相乘:

2 \cdot 5 = 10

= \frac{- 10}{10}

使用分式法则:

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

= - \frac{10}{10}

使用法则

\frac{a}{a} = 1

= - 1

二次方程组的解是:

u = \frac{2}{5}, u = - 1

u = cos (x)

代回

cos (x) = \frac{2}{5}, cos (x) = - 1

cos (x) = \frac{2}{5}, cos (x) = - 1

cos (x) = \frac{2}{5} : x = arccos (\frac{2}{5}) + 2 πn, x = 2 π - arccos (\frac{2}{5}) + 2 πn

cos (x) = \frac{2}{5}

使用反三角函数性质

cos (x) = \frac{2}{5}

cos (x) = \frac{2}{5}

的通解

cos (x) = a \Rightarrow x = arccos (a) + 2 πn, x = 2 π - arccos (a) + 2 πn

x = arccos (\frac{2}{5}) + 2 πn, x = 2 π - arccos (\frac{2}{5}) + 2 πn

x = arccos (\frac{2}{5}) + 2 πn, x = 2 π - arccos (\frac{2}{5}) + 2 πn

cos (x) = - 1 : x = π + 2 πn

cos (x) = - 1

cos (x) = - 1

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}

x = π + 2 πn

x = π + 2 πn

合并所有解

x = arccos (\frac{2}{5}) + 2 πn, x = 2 π - arccos (\frac{2}{5}) + 2 πn, x = π + 2 πn

以小数形式表示解

x = 1.15927 \dots + 2 πn, x = 2 π - 1.15927 \dots + 2 πn, x = π + 2 πn

作图

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