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受欢迎的三角函数 >

sin(a/2)=(1/2)sin(a)

解答

sin (\frac{a}{2}) = (\frac{1}{2}) sin (a)

解答

a = 4 πn, a = 2 π + 4 πn

+1

度数

a = 0^{\circ} + 72 0^{\circ} n, a = 36 0^{\circ} + 72 0^{\circ} n

求解步骤

sin (\frac{a}{2}) = (\frac{1}{2}) sin (a)

两边减去

\frac{1}{2} sin (a)

sin (\frac{a}{2}) - \frac{1}{2} sin (a) = 0

化简

sin (\frac{a}{2}) - \frac{1}{2} sin (a) : \frac{2 sin ( \frac{a}{2} ) - sin ( a )}{2}

sin (\frac{a}{2}) - \frac{1}{2} sin (a)

\frac{1}{2} sin (a) = \frac{sin ( a )}{2}

\frac{1}{2} sin (a)

分式相乘:

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

= \frac{1 \cdot sin ( a )}{2}

乘以:

1 \cdot sin (a) = sin (a)

= \frac{sin ( a )}{2}

= sin (\frac{a}{2}) - \frac{sin ( a )}{2}

将项转换为分式:

sin (\frac{a}{2}) = \frac{sin ( \frac{a}{2} ) 2}{2}

= \frac{sin ( \frac{a}{2} ) \cdot 2}{2} - \frac{sin ( a )}{2}

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 ( \frac{a}{2} ) \cdot 2 - sin ( a )}{2}

\frac{2 sin ( \frac{a}{2} ) - sin ( a )}{2} = 0

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

2 sin (\frac{a}{2}) - sin (a) = 0

令

: u = \frac{a}{2}

2 sin (u) - sin (2 u) = 0

使用三角恒等式改写

- sin (2 u) + 2 sin (u)

使用倍角公式:

sin (2 x) = 2 sin (x) cos (x)

= - 2 sin (u) cos (u) + 2 sin (u)

2 sin (u) - 2 cos (u) sin (u) = 0

分解

2 sin (u) - 2 cos (u) sin (u) : - 2 sin (u) (cos (u) - 1)

2 sin (u) - 2 cos (u) sin (u)

因式分解出通项

- 2 sin (u)

= - 2 sin (u) (- 1 + cos (u))

- 2 sin (u) (cos (u) - 1) = 0

分别求解每个部分

sin (u) = 0 or cos (u) - 1 = 0

sin (u) = 0 : u = 2 πn, u = π + 2 πn

sin (u) = 0

sin (u) = 0

的通解

sin (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} sin (x) 0 \frac{1}{2} \frac{2}{2} \frac{3}{2} 1 \frac{3}{2} \frac{2}{2} \frac{1}{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} sin (x) 0 - \frac{1}{2} - \frac{2}{2} - \frac{3}{2} - 1 - \frac{3}{2} - \frac{2}{2} - \frac{1}{2}

u = 0 + 2 πn, u = π + 2 πn

u = 0 + 2 πn, u = π + 2 πn

解

u = 0 + 2 πn : u = 2 πn

u = 0 + 2 πn

0 + 2 πn = 2 πn

u = 2 πn

u = 2 πn, u = π + 2 πn

cos (u) - 1 = 0 : u = 2 πn

cos (u) - 1 = 0

将

1

到右边

cos (u) - 1 = 0

两边加上

1

cos (u) - 1 + 1 = 0 + 1

化简

cos (u) = 1

cos (u) = 1

cos (u) = 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}

u = 0 + 2 πn

u = 0 + 2 πn

解

u = 0 + 2 πn : u = 2 πn

u = 0 + 2 πn

0 + 2 πn = 2 πn

u = 2 πn

u = 2 πn

合并所有解

u = 2 πn, u = π + 2 πn

u = \frac{a}{2}

代回

\frac{a}{2} = 2 πn : a = 4 πn

\frac{a}{2} = 2 πn

在两边乘以

2

\frac{a}{2} = 2 πn

在两边乘以

2

\frac{2 a}{2} = 2 \cdot 2 πn

化简

a = 4 πn

a = 4 πn

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

\frac{a}{2} = π + 2 πn

在两边乘以

2

\frac{a}{2} = π + 2 πn

在两边乘以

2

\frac{2 a}{2} = 2 π + 2 \cdot 2 πn

化简

a = 2 π + 4 πn

a = 2 π + 4 πn

a = 4 πn, a = 2 π + 4 πn

作图

流行的例子

cos(x/2)=cos(x)+1

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

d=(sin^4(x)-cos^2(x)+5)/(4*cos^2(x))

d = \frac{sin ^{4} ( x ) - cos ^{2} ( x ) + 5}{4 \cdot cos ^{2} ( x )}

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

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

tan^3(3x)-2sin^3(3x)=0

tan^{3} (3 x) - 2 sin^{3} (3 x) = 0

cot^5(x)=(-1)/((sqrt(3)))

cot^{5} (x) = \frac{- 1}{( 3 )}