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受欢迎的三角函数 >

sin^2(x)-cos(x)= 1/4

解答

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

解答

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

+1

度数

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

求解步骤

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

两边减去

\frac{1}{4}

sin^{2} (x) - cos (x) - \frac{1}{4} = 0

化简

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

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

将项转换为分式:

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

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

因为分母相等，所以合并分式:

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

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

\frac{4 sin ^{2} ( x ) - 4 cos ( x ) - 1}{4} = 0

\frac{f ( x )}{g ( x )} = 0 \Rightarrow f (x) = 0

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

使用三角恒等式改写

- 1 - 4 cos (x) + 4 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)

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

化简

- 1 - 4 cos (x) + 4 (1 - cos^{2} (x)) : - 4 cos^{2} (x) - 4 cos (x) + 3

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

数字相乘:

4 \cdot 1 = 4

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

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

化简

- 1 - 4 cos (x) + 4 - 4 cos^{2} (x) : - 4 cos^{2} (x) - 4 cos (x) + 3

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

对同类项分组

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

数字相加/相减:

- 1 + 4 = 3

= - 4 cos^{2} (x) - 4 cos (x) + 3

= - 4 cos^{2} (x) - 4 cos (x) + 3

= - 4 cos^{2} (x) - 4 cos (x) + 3

3 - 4 cos (x) - 4 cos^{2} (x) = 0

用替代法求解

3 - 4 cos (x) - 4 cos^{2} (x) = 0

令

: cos (x) = u

3 - 4 u - 4 u^{2} = 0

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

3 - 4 u - 4 u^{2} = 0

改写成标准形式

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

- 4 u^{2} - 4 u + 3 = 0

使用求根公式求解

- 4 u^{2} - 4 u + 3 = 0

二次方程求根公式:

若

a = - 4, b = - 4, c = 3

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

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

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

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

使用法则

- (- a) = a

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

使用指数法则:

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

若

n

是偶数

(- 4)^{2} = 4^{2}

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

数字相乘:

4 \cdot 4 \cdot 3 = 48

= 4^{2} + 48

4^{2} = 16

= 16 + 48

数字相加:

16 + 48 = 64

= 64

因式分解数字:

64 = 8^{2}

= 8^{2}

使用根式运算法则:

n a^{n} = a

8^{2} = 8

= 8

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

将解分隔开

u_{1} = \frac{- ( - 4 ) + 8}{2 ( - 4 )}, u_{2} = \frac{- ( - 4 ) - 8}{2 ( - 4 )}

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

\frac{- ( - 4 ) + 8}{2 ( - 4 )}

去除括号:

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

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

数字相加:

4 + 8 = 12

= \frac{12}{- 2 \cdot 4}

数字相乘:

2 \cdot 4 = 8

= \frac{12}{- 8}

使用分式法则:

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

= - \frac{12}{8}

约分:

4

= - \frac{3}{2}

u = \frac{- ( - 4 ) - 8}{2 ( - 4 )} : \frac{1}{2}

\frac{- ( - 4 ) - 8}{2 ( - 4 )}

去除括号:

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

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

数字相减:

4 - 8 = - 4

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

数字相乘:

2 \cdot 4 = 8

= \frac{- 4}{- 8}

使用分式法则:

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

= \frac{4}{8}

约分:

4

= \frac{1}{2}

二次方程组的解是:

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

u = cos (x)

代回

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

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

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

无解

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

- 1 \leq cos (x) \leq 1

无解

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

作图

流行的例子

cos^4(a)=3+4cos^2(a)+cos^4(a)

cos^{4} (a) = 3 + 4 cos^{2} (a) + cos^{4} (a)

cos^{4} (t) = 1

8cos^2(x)-12sin(x)-12=0

8 cos^{2} (x) - 12 sin (x) - 12 = 0

1 - cos (x) = 2

8sin^2(x)+6cos^2(x)=10

8 sin^{2} (x) + 6 cos^{2} (x) = 10