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受欢迎的三角函数 >

5-5cos(x)=4sin^2(x)

解答

5 - 5 cos (x) = 4 sin^{2} (x)

解答

x = 2 πn, x = 1.31811 \dots + 2 πn, x = 2 π - 1.31811 \dots + 2 πn

+1

度数

x = 0^{\circ} + 36 0^{\circ} n, x = 75.52248 \dots^{\circ} + 36 0^{\circ} n, x = 284.47751 \dots^{\circ} + 36 0^{\circ} n

求解步骤

5 - 5 cos (x) = 4 sin^{2} (x)

两边减去

4 sin^{2} (x)

5 - 5 cos (x) - 4 sin^{2} (x) = 0

使用三角恒等式改写

5 - 4 sin^{2} (x) - 5 cos (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 - 4 (1 - cos^{2} (x)) - 5 cos (x)

化简

5 - 4 (1 - cos^{2} (x)) - 5 cos (x) : 4 cos^{2} (x) - 5 cos (x) + 1

5 - 4 (1 - cos^{2} (x)) - 5 cos (x)

乘开

- 4 (1 - cos^{2} (x)) : - 4 + 4 cos^{2} (x)

- 4 (1 - cos^{2} (x))

使用分配律:

a (b - c) = ab - a c

a = - 4, b = 1, c = cos^{2} (x)

= - 4 \cdot 1 - (- 4) cos^{2} (x)

使用加减运算法则

- (- a) = a

= - 4 \cdot 1 + 4 cos^{2} (x)

数字相乘:

4 \cdot 1 = 4

= - 4 + 4 cos^{2} (x)

= 5 - 4 + 4 cos^{2} (x) - 5 cos (x)

数字相减:

5 - 4 = 1

= 4 cos^{2} (x) - 5 cos (x) + 1

= 4 cos^{2} (x) - 5 cos (x) + 1

1 + 4 cos^{2} (x) - 5 cos (x) = 0

用替代法求解

1 + 4 cos^{2} (x) - 5 cos (x) = 0

令

: cos (x) = u

1 + 4 u^{2} - 5 u = 0

1 + 4 u^{2} - 5 u = 0 : u = 1, u = \frac{1}{4}

1 + 4 u^{2} - 5 u = 0

改写成标准形式

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

4 u^{2} - 5 u + 1 = 0

使用求根公式求解

4 u^{2} - 5 u + 1 = 0

二次方程求根公式:

若

a = 4, b = - 5, c = 1

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

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

(- 5)^{2} - 4 \cdot 4 \cdot 1 = 3

(- 5)^{2} - 4 \cdot 4 \cdot 1

使用指数法则:

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

若

n

是偶数

(- 5)^{2} = 5^{2}

= 5^{2} - 4 \cdot 4 \cdot 1

数字相乘:

4 \cdot 4 \cdot 1 = 16

= 5^{2} - 16

5^{2} = 25

= 25 - 16

数字相减:

25 - 16 = 9

= 9

因式分解数字:

9 = 3^{2}

= 3^{2}

使用根式运算法则:

n a^{n} = a

3^{2} = 3

= 3

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

将解分隔开

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

u = \frac{- ( - 5 ) + 3}{2 \cdot 4} : 1

\frac{- ( - 5 ) + 3}{2 \cdot 4}

使用法则

- (- a) = a

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

数字相加:

5 + 3 = 8

= \frac{8}{2 \cdot 4}

数字相乘:

2 \cdot 4 = 8

= \frac{8}{8}

使用法则

\frac{a}{a} = 1

= 1

u = \frac{- ( - 5 ) - 3}{2 \cdot 4} : \frac{1}{4}

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

使用法则

- (- a) = a

= \frac{5 - 3}{2 \cdot 4}

数字相减:

5 - 3 = 2

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

数字相乘:

2 \cdot 4 = 8

= \frac{2}{8}

约分:

2

= \frac{1}{4}

二次方程组的解是:

u = 1, u = \frac{1}{4}

u = cos (x)

代回

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

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

cos (x) = 1 : x = 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 = 0 + 2 πn

x = 0 + 2 πn

解

x = 0 + 2 πn : x = 2 πn

x = 0 + 2 πn

0 + 2 πn = 2 πn

x = 2 πn

x = 2 πn

cos (x) = \frac{1}{4} : x = arccos (\frac{1}{4}) + 2 πn, x = 2 π - arccos (\frac{1}{4}) + 2 πn

cos (x) = \frac{1}{4}

使用反三角函数性质

cos (x) = \frac{1}{4}

cos (x) = \frac{1}{4}

的通解

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

x = arccos (\frac{1}{4}) + 2 πn, x = 2 π - arccos (\frac{1}{4}) + 2 πn

x = arccos (\frac{1}{4}) + 2 πn, x = 2 π - arccos (\frac{1}{4}) + 2 πn

合并所有解

x = 2 πn, x = arccos (\frac{1}{4}) + 2 πn, x = 2 π - arccos (\frac{1}{4}) + 2 πn

以小数形式表示解

x = 2 πn, x = 1.31811 \dots + 2 πn, x = 2 π - 1.31811 \dots + 2 πn

作图

流行的例子

3cot^2(x)-14csc(x)-2=0

3 cot^{2} (x) - 14 csc (x) - 2 = 0

250=100*5*cos(x)

250 = 100 \cdot 5 \cdot cos (x)

tan(x)= 1/(sin(2x)),0<= x<= pi

tan (x) = \frac{1}{sin ( 2 x )}, 0 \leq x \leq π

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

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cos (x) = \frac{1}{4} - sin^{2} (x)