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受欢迎的三角函数 >

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

解答

8 sin^{2} (x) - 2 cos (x) = 5

解答

x = 2.41885 \dots + 2 πn, x = - 2.41885 \dots + 2 πn, x = \frac{π}{3} + 2 πn, x = \frac{5 π}{3} + 2 πn

+1

度数

x = 138.59037 \dots^{\circ} + 36 0^{\circ} n, x = - 138.59037 \dots^{\circ} + 36 0^{\circ} n, x = 6 0^{\circ} + 36 0^{\circ} n, x = 30 0^{\circ} + 36 0^{\circ} n

求解步骤

8 sin^{2} (x) - 2 cos (x) = 5

两边减去

5

8 sin^{2} (x) - 2 cos (x) - 5 = 0

使用三角恒等式改写

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

使用毕达哥拉斯恒等式:

cos^{2} (x) + sin^{2} (x) = 1

sin^{2} (x) = 1 - cos^{2} (x)

= - 5 - 2 cos (x) + 8 (1 - cos^{2} (x))

化简

- 5 - 2 cos (x) + 8 (1 - cos^{2} (x)) : - 8 cos^{2} (x) - 2 cos (x) + 3

- 5 - 2 cos (x) + 8 (1 - cos^{2} (x))

乘开

8 (1 - cos^{2} (x)) : 8 - 8 cos^{2} (x)

8 (1 - cos^{2} (x))

使用分配律:

a (b - c) = ab - a c

a = 8, b = 1, c = cos^{2} (x)

= 8 \cdot 1 - 8 cos^{2} (x)

数字相乘:

8 \cdot 1 = 8

= 8 - 8 cos^{2} (x)

= - 5 - 2 cos (x) + 8 - 8 cos^{2} (x)

化简

- 5 - 2 cos (x) + 8 - 8 cos^{2} (x) : - 8 cos^{2} (x) - 2 cos (x) + 3

- 5 - 2 cos (x) + 8 - 8 cos^{2} (x)

对同类项分组

= - 2 cos (x) - 8 cos^{2} (x) - 5 + 8

数字相加/相减:

- 5 + 8 = 3

= - 8 cos^{2} (x) - 2 cos (x) + 3

= - 8 cos^{2} (x) - 2 cos (x) + 3

= - 8 cos^{2} (x) - 2 cos (x) + 3

3 - 2 cos (x) - 8 cos^{2} (x) = 0

用替代法求解

3 - 2 cos (x) - 8 cos^{2} (x) = 0

令

: cos (x) = u

3 - 2 u - 8 u^{2} = 0

3 - 2 u - 8 u^{2} = 0 : u = - \frac{3}{4}, u = \frac{1}{2}

3 - 2 u - 8 u^{2} = 0

改写成标准形式

a x^{2} + b x + c = 0

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

使用求根公式求解

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

二次方程求根公式:

若

a = - 8, b = - 2, c = 3

u_{1, 2} = \frac{- ( - 2 ) \pm ( - 2 ) ^{2} - 4 ( - 8 ) \cdot 3}{2 ( - 8 )}

u_{1, 2} = \frac{- ( - 2 ) \pm ( - 2 ) ^{2} - 4 ( - 8 ) \cdot 3}{2 ( - 8 )}

(- 2)^{2} - 4 (- 8) \cdot 3 = 10

(- 2)^{2} - 4 (- 8) \cdot 3

使用法则

- (- a) = a

= (- 2)^{2} + 4 \cdot 8 \cdot 3

使用指数法则:

(- a)^{n} = a^{n},

若

n

是偶数

(- 2)^{2} = 2^{2}

= 2^{2} + 4 \cdot 8 \cdot 3

数字相乘:

4 \cdot 8 \cdot 3 = 96

= 2^{2} + 96

2^{2} = 4

= 4 + 96

数字相加:

4 + 96 = 100

= 100

因式分解数字:

100 = 1 0^{2}

= 1 0^{2}

使用根式运算法则:

n a^{n} = a

1 0^{2} = 10

= 10

u_{1, 2} = \frac{- ( - 2 ) \pm 10}{2 ( - 8 )}

将解分隔开

u_{1} = \frac{- ( - 2 ) + 10}{2 ( - 8 )}, u_{2} = \frac{- ( - 2 ) - 10}{2 ( - 8 )}

u = \frac{- ( - 2 ) + 10}{2 ( - 8 )} : - \frac{3}{4}

\frac{- ( - 2 ) + 10}{2 ( - 8 )}

去除括号:

(- a) = - a, - (- a) = a

= \frac{2 + 10}{- 2 \cdot 8}

数字相加:

2 + 10 = 12

= \frac{12}{- 2 \cdot 8}

数字相乘:

2 \cdot 8 = 16

= \frac{12}{- 16}

使用分式法则:

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

= - \frac{12}{16}

约分:

4

= - \frac{3}{4}

u = \frac{- ( - 2 ) - 10}{2 ( - 8 )} : \frac{1}{2}

\frac{- ( - 2 ) - 10}{2 ( - 8 )}

去除括号:

(- a) = - a, - (- a) = a

= \frac{2 - 10}{- 2 \cdot 8}

数字相减:

2 - 10 = - 8

= \frac{- 8}{- 2 \cdot 8}

数字相乘:

2 \cdot 8 = 16

= \frac{- 8}{- 16}

使用分式法则:

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

= \frac{8}{16}

约分:

8

= \frac{1}{2}

二次方程组的解是:

u = - \frac{3}{4}, u = \frac{1}{2}

u = cos (x)

代回

cos (x) = - \frac{3}{4}, cos (x) = \frac{1}{2}

cos (x) = - \frac{3}{4}, cos (x) = \frac{1}{2}

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

cos (x) = - \frac{3}{4}

使用反三角函数性质

cos (x) = - \frac{3}{4}

cos (x) = - \frac{3}{4}

的通解

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

x = arccos (- \frac{3}{4}) + 2 πn, x = - arccos (- \frac{3}{4}) + 2 πn

x = arccos (- \frac{3}{4}) + 2 πn, x = - arccos (- \frac{3}{4}) + 2 πn

cos (x) = \frac{1}{2} : x = \frac{π}{3} + 2 πn, x = \frac{5 π}{3} + 2 πn

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

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

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

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

合并所有解

x = arccos (- \frac{3}{4}) + 2 πn, x = - arccos (- \frac{3}{4}) + 2 πn, x = \frac{π}{3} + 2 πn, x = \frac{5 π}{3} + 2 πn

以小数形式表示解

x = 2.41885 \dots + 2 πn, x = - 2.41885 \dots + 2 πn, x = \frac{π}{3} + 2 πn, x = \frac{5 π}{3} + 2 πn

作图

流行的例子

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22 = 18 + 8 cos (\frac{x + 9}{12} π)

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