解答
sin(2x)−sin(4x)+sin(6x)=0
解答
x=πn,x=2π+2πn,x=6π+6πn,x=65π+6πn,x=4π+4πn,x=43π+4πn
+1
度数
x=0∘+180∘n,x=90∘+180∘n,x=30∘+180∘n,x=150∘+180∘n,x=45∘+180∘n,x=135∘+180∘n求解步骤
sin(2x)−sin(4x)+sin(6x)=0
令:u=2xsin(u)−sin(2u)+sin(3u)=0
使用三角恒等式改写
−sin(2u)+sin(3u)+sin(u)
使用倍角公式: sin(2x)=2sin(x)cos(x)=−2sin(u)cos(u)+sin(3u)+sin(u)
sin(3u)=3sin(u)−4sin3(u)
sin(3u)
使用三角恒等式改写
sin(3u)
改写为=sin(2u+u)
使用角和恒等式: sin(s+t)=sin(s)cos(t)+cos(s)sin(t)=sin(2u)cos(u)+cos(2u)sin(u)
使用倍角公式: sin(2u)=2sin(u)cos(u)=cos(2u)sin(u)+cos(u)2sin(u)cos(u)
化简 cos(2u)sin(u)+cos(u)⋅2sin(u)cos(u):sin(u)cos(2u)+2cos2(u)sin(u)
cos(2u)sin(u)+cos(u)2sin(u)cos(u)
cos(u)⋅2sin(u)cos(u)=2cos2(u)sin(u)
cos(u)2sin(u)cos(u)
使用指数法则: ab⋅ac=ab+ccos(u)cos(u)=cos1+1(u)=2sin(u)cos1+1(u)
数字相加:1+1=2=2sin(u)cos2(u)
=sin(u)cos(2u)+2cos2(u)sin(u)
=sin(u)cos(2u)+2cos2(u)sin(u)
=sin(u)cos(2u)+2cos2(u)sin(u)
使用倍角公式: cos(2u)=1−2sin2(u)=(1−2sin2(u))sin(u)+2cos2(u)sin(u)
使用毕达哥拉斯恒等式: cos2(u)+sin2(u)=1cos2(u)=1−sin2(u)=(1−2sin2(u))sin(u)+2(1−sin2(u))sin(u)
乘开 (1−2sin2(u))sin(u)+2(1−sin2(u))sin(u):−4sin3(u)+3sin(u)
(1−2sin2(u))sin(u)+2(1−sin2(u))sin(u)
=sin(u)(1−2sin2(u))+2sin(u)(1−sin2(u))
乘开 sin(u)(1−2sin2(u)):sin(u)−2sin3(u)
sin(u)(1−2sin2(u))
使用分配律: a(b−c)=ab−aca=sin(u),b=1,c=2sin2(u)=sin(u)1−sin(u)2sin2(u)
=1sin(u)−2sin2(u)sin(u)
化简 1⋅sin(u)−2sin2(u)sin(u):sin(u)−2sin3(u)
1sin(u)−2sin2(u)sin(u)
1⋅sin(u)=sin(u)
1sin(u)
乘以:1⋅sin(u)=sin(u)=sin(u)
2sin2(u)sin(u)=2sin3(u)
2sin2(u)sin(u)
使用指数法则: ab⋅ac=ab+csin2(u)sin(u)=sin2+1(u)=2sin2+1(u)
数字相加:2+1=3=2sin3(u)
=sin(u)−2sin3(u)
=sin(u)−2sin3(u)
=sin(u)−2sin3(u)+2(1−sin2(u))sin(u)
乘开 2sin(u)(1−sin2(u)):2sin(u)−2sin3(u)
2sin(u)(1−sin2(u))
使用分配律: a(b−c)=ab−aca=2sin(u),b=1,c=sin2(u)=2sin(u)1−2sin(u)sin2(u)
=2⋅1sin(u)−2sin2(u)sin(u)
化简 2⋅1⋅sin(u)−2sin2(u)sin(u):2sin(u)−2sin3(u)
2⋅1sin(u)−2sin2(u)sin(u)
2⋅1⋅sin(u)=2sin(u)
2⋅1sin(u)
数字相乘:2⋅1=2=2sin(u)
2sin2(u)sin(u)=2sin3(u)
2sin2(u)sin(u)
使用指数法则: ab⋅ac=ab+csin2(u)sin(u)=sin2+1(u)=2sin2+1(u)
数字相加:2+1=3=2sin3(u)
=2sin(u)−2sin3(u)
=2sin(u)−2sin3(u)
=sin(u)−2sin3(u)+2sin(u)−2sin3(u)
化简 sin(u)−2sin3(u)+2sin(u)−2sin3(u):−4sin3(u)+3sin(u)
sin(u)−2sin3(u)+2sin(u)−2sin3(u)
对同类项分组=−2sin3(u)−2sin3(u)+sin(u)+2sin(u)
同类项相加:−2sin3(u)−2sin3(u)=−4sin3(u)=−4sin3(u)+sin(u)+2sin(u)
同类项相加:sin(u)+2sin(u)=3sin(u)=−4sin3(u)+3sin(u)
=−4sin3(u)+3sin(u)
=−4sin3(u)+3sin(u)
=3sin(u)−4sin3(u)+sin(u)−2cos(u)sin(u)
化简=4sin(u)−4sin3(u)−2cos(u)sin(u)
4sin(u)−4sin3(u)−2cos(u)sin(u)=0
分解 4sin(u)−4sin3(u)−2cos(u)sin(u):2sin(u)(2−2sin2(u)−cos(u))
4sin(u)−4sin3(u)−2cos(u)sin(u)
使用指数法则: ab+c=abacsin3(u)=sin(u)sin2(u)=4sin(u)−4sin(u)sin2(u)−2sin(u)cos(u)
将 −4 改写为 2⋅2将 4 改写为 2⋅2=2⋅2sin(u)+2⋅2sin(u)sin2(u)−2sin(u)cos(u)
因式分解出通项 2sin(u)=2sin(u)(2−2sin2(u)−cos(u))
2sin(u)(2−2sin2(u)−cos(u))=0
分别求解每个部分sin(u)=0or2−2sin2(u)−cos(u)=0
sin(u)=0:u=2πn,u=π+2πn
sin(u)=0
sin(u)=0的通解
sin(x) 周期表(周期为 2πn"):
x06π4π3π2π32π43π65πsin(x)02122231232221xπ67π45π34π23π35π47π611πsin(x)0−21−22−23−1−23−22−21
u=0+2πn,u=π+2πn
u=0+2πn,u=π+2πn
解 u=0+2πn:u=2πn
u=0+2πn
0+2πn=2πnu=2πn
u=2πn,u=π+2πn
2−2sin2(u)−cos(u)=0:u=3π+2πn,u=35π+2πn,u=2π+2πn,u=23π+2πn
2−2sin2(u)−cos(u)=0
使用三角恒等式改写
2−cos(u)−2sin2(u)
使用毕达哥拉斯恒等式: cos2(x)+sin2(x)=1sin2(x)=1−cos2(x)=2−cos(u)−2(1−cos2(u))
化简 2−cos(u)−2(1−cos2(u)):2cos2(u)−cos(u)
2−cos(u)−2(1−cos2(u))
乘开 −2(1−cos2(u)):−2+2cos2(u)
−2(1−cos2(u))
使用分配律: a(b−c)=ab−aca=−2,b=1,c=cos2(u)=−2⋅1−(−2)cos2(u)
使用加减运算法则−(−a)=a=−2⋅1+2cos2(u)
数字相乘:2⋅1=2=−2+2cos2(u)
=2−cos(u)−2+2cos2(u)
化简 2−cos(u)−2+2cos2(u):2cos2(u)−cos(u)
2−cos(u)−2+2cos2(u)
对同类项分组=−cos(u)+2cos2(u)+2−2
2−2=0=2cos2(u)−cos(u)
=2cos2(u)−cos(u)
=2cos2(u)−cos(u)
−cos(u)+2cos2(u)=0
用替代法求解
−cos(u)+2cos2(u)=0
令:cos(u)=u−u+2u2=0
−u+2u2=0:u=21,u=0
−u+2u2=0
改写成标准形式 ax2+bx+c=02u2−u=0
使用求根公式求解
2u2−u=0
二次方程求根公式:
若 a=2,b=−1,c=0u1,2=2⋅2−(−1)±(−1)2−4⋅2⋅0
u1,2=2⋅2−(−1)±(−1)2−4⋅2⋅0
(−1)2−4⋅2⋅0=1
(−1)2−4⋅2⋅0
(−1)2=1
(−1)2
使用指数法则: (−a)n=an,若 n 是偶数(−1)2=12=12
使用法则 1a=1=1
4⋅2⋅0=0
4⋅2⋅0
使用法则 0⋅a=0=0
=1−0
数字相减:1−0=1=1
使用法则 1=1=1
u1,2=2⋅2−(−1)±1
将解分隔开u1=2⋅2−(−1)+1,u2=2⋅2−(−1)−1
u=2⋅2−(−1)+1:21
2⋅2−(−1)+1
使用法则 −(−a)=a=2⋅21+1
数字相加:1+1=2=2⋅22
数字相乘:2⋅2=4=42
约分:2=21
u=2⋅2−(−1)−1:0
2⋅2−(−1)−1
使用法则 −(−a)=a=2⋅21−1
数字相减:1−1=0=2⋅20
数字相乘:2⋅2=4=40
使用法则 a0=0,a=0=0
二次方程组的解是:u=21,u=0
u=cos(u)代回cos(u)=21,cos(u)=0
cos(u)=21,cos(u)=0
cos(u)=21:u=3π+2πn,u=35π+2πn
cos(u)=21
cos(u)=21的通解
cos(x) 周期表(周期为 2πn):
x06π4π3π2π32π43π65πcos(x)12322210−21−22−23xπ67π45π34π23π35π47π611πcos(x)−1−23−22−210212223
u=3π+2πn,u=35π+2πn
u=3π+2πn,u=35π+2πn
cos(u)=0:u=2π+2πn,u=23π+2πn
cos(u)=0
cos(u)=0的通解
cos(x) 周期表(周期为 2πn):
x06π4π3π2π32π43π65πcos(x)12322210−21−22−23xπ67π45π34π23π35π47π611πcos(x)−1−23−22−210212223
u=2π+2πn,u=23π+2πn
u=2π+2πn,u=23π+2πn
合并所有解u=3π+2πn,u=35π+2πn,u=2π+2πn,u=23π+2πn
合并所有解u=2πn,u=π+2πn,u=3π+2πn,u=35π+2πn,u=2π+2πn,u=23π+2πn
u=2x代回
2x=2πn:x=πn
2x=2πn
两边除以 2
2x=2πn
两边除以 222x=22πn
化简x=πn
x=πn
2x=π+2πn:x=2π+2πn
2x=π+2πn
两边除以 2
2x=π+2πn
两边除以 222x=2π+22πn
化简
22x=2π+22πn
化简 22x:x
22x
数字相除:22=1=x
化简 2π+22πn:2π+2πn
2π+22πn
使用法则 ca±cb=ca±b=2π+2πn
x=2π+2πn
x=2π+2πn
x=2π+2πn
2x=3π+2πn:x=6π+6πn
2x=3π+2πn
两边除以 2
2x=3π+2πn
两边除以 222x=23π+22πn
化简
22x=23π+22πn
化简 22x:x
22x
数字相除:22=1=x
化简 23π+22πn:6π+6πn
23π+22πn
使用法则 ca±cb=ca±b=23π+2πn
化简 3π+2πn:3π+6πn
3π+2πn
将项转换为分式: 2πn=32πn3=3π+32πn⋅3
因为分母相等,所以合并分式: ca±cb=ca±b=3π+2πn⋅3
数字相乘:2⋅3=6=3π+6πn
=23π+6πn
使用分式法则: acb=c⋅ab=3⋅2π+6πn
数字相乘:3⋅2=6=6π+6πn
x=6π+6πn
x=6π+6πn
x=6π+6πn
2x=35π+2πn:x=65π+6πn
2x=35π+2πn
两边除以 2
2x=35π+2πn
两边除以 222x=235π+22πn
化简
22x=235π+22πn
化简 22x:x
22x
数字相除:22=1=x
化简 235π+22πn:65π+6πn
235π+22πn
使用法则 ca±cb=ca±b=235π+2πn
化简 35π+2πn:35π+6πn
35π+2πn
将项转换为分式: 2πn=32πn3=35π+32πn⋅3
因为分母相等,所以合并分式: ca±cb=ca±b=35π+2πn⋅3
数字相乘:2⋅3=6=35π+6πn
=235π+6πn
使用分式法则: acb=c⋅ab=3⋅25π+6πn
数字相乘:3⋅2=6=65π+6πn
x=65π+6πn
x=65π+6πn
x=65π+6πn
2x=2π+2πn:x=4π+4πn
2x=2π+2πn
两边除以 2
2x=2π+2πn
两边除以 222x=22π+22πn
化简
22x=22π+22πn
化简 22x:x
22x
数字相除:22=1=x
化简 22π+22πn:4π+4πn
22π+22πn
使用法则 ca±cb=ca±b=22π+2πn
化简 2π+2πn:2π+4πn
2π+2πn
将项转换为分式: 2πn=22πn2=2π+22πn⋅2
因为分母相等,所以合并分式: ca±cb=ca±b=2π+2πn⋅2
数字相乘:2⋅2=4=2π+4πn
=22π+4πn
使用分式法则: acb=c⋅ab=2⋅2π+4πn
数字相乘:2⋅2=4=4π+4πn
x=4π+4πn
x=4π+4πn
x=4π+4πn
2x=23π+2πn:x=43π+4πn
2x=23π+2πn
两边除以 2
2x=23π+2πn
两边除以 222x=223π+22πn
化简
22x=223π+22πn
化简 22x:x
22x
数字相除:22=1=x
化简 223π+22πn:43π+4πn
223π+22πn
使用法则 ca±cb=ca±b=223π+2πn
化简 23π+2πn:23π+4πn
23π+2πn
将项转换为分式: 2πn=22πn2=23π+22πn⋅2
因为分母相等,所以合并分式: ca±cb=ca±b=23π+2πn⋅2
数字相乘:2⋅2=4=23π+4πn
=223π+4πn
使用分式法则: acb=c⋅ab=2⋅23π+4πn
数字相乘:2⋅2=4=43π+4πn
x=43π+4πn
x=43π+4πn
x=43π+4πn
x=πn,x=2π+2πn,x=6π+6πn,x=65π+6πn,x=4π+4πn,x=43π+4πn