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Popular Trigonometry >

sin(2x)-sin(4x)+sin(6x)=0

  • Pre Algebra
  • Algebra
  • Pre Calculus
  • Calculus
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Solution

sin(2x)−sin(4x)+sin(6x)=0

Solution

x=πn,x=2π+2πn​,x=6π+6πn​,x=65π+6πn​,x=4π+4πn​,x=43π+4πn​
+1
Degrees
x=0∘+180∘n,x=90∘+180∘n,x=30∘+180∘n,x=150∘+180∘n,x=45∘+180∘n,x=135∘+180∘n
Solution steps
sin(2x)−sin(4x)+sin(6x)=0
Let: u=2xsin(u)−sin(2u)+sin(3u)=0
Rewrite using trig identities
−sin(2u)+sin(3u)+sin(u)
Use the Double Angle identity: sin(2x)=2sin(x)cos(x)=−2sin(u)cos(u)+sin(3u)+sin(u)
sin(3u)=3sin(u)−4sin3(u)
sin(3u)
Rewrite using trig identities
sin(3u)
Rewrite as=sin(2u+u)
Use the Angle Sum identity: sin(s+t)=sin(s)cos(t)+cos(s)sin(t)=sin(2u)cos(u)+cos(2u)sin(u)
Use the Double Angle identity: sin(2u)=2sin(u)cos(u)=cos(2u)sin(u)+cos(u)2sin(u)cos(u)
Simplify 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)
Apply exponent rule: ab⋅ac=ab+ccos(u)cos(u)=cos1+1(u)=2sin(u)cos1+1(u)
Add the numbers: 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)
Use the Double Angle identity: cos(2u)=1−2sin2(u)=(1−2sin2(u))sin(u)+2cos2(u)sin(u)
Use the Pythagorean identity: cos2(u)+sin2(u)=1cos2(u)=1−sin2(u)=(1−2sin2(u))sin(u)+2(1−sin2(u))sin(u)
Expand (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))
Expand sin(u)(1−2sin2(u)):sin(u)−2sin3(u)
sin(u)(1−2sin2(u))
Apply the distributive law: 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)
Simplify 1⋅sin(u)−2sin2(u)sin(u):sin(u)−2sin3(u)
1sin(u)−2sin2(u)sin(u)
1⋅sin(u)=sin(u)
1sin(u)
Multiply: 1⋅sin(u)=sin(u)=sin(u)
2sin2(u)sin(u)=2sin3(u)
2sin2(u)sin(u)
Apply exponent rule: ab⋅ac=ab+csin2(u)sin(u)=sin2+1(u)=2sin2+1(u)
Add the numbers: 2+1=3=2sin3(u)
=sin(u)−2sin3(u)
=sin(u)−2sin3(u)
=sin(u)−2sin3(u)+2(1−sin2(u))sin(u)
Expand 2sin(u)(1−sin2(u)):2sin(u)−2sin3(u)
2sin(u)(1−sin2(u))
Apply the distributive law: 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)
Simplify 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)
Multiply the numbers: 2⋅1=2=2sin(u)
2sin2(u)sin(u)=2sin3(u)
2sin2(u)sin(u)
Apply exponent rule: ab⋅ac=ab+csin2(u)sin(u)=sin2+1(u)=2sin2+1(u)
Add the numbers: 2+1=3=2sin3(u)
=2sin(u)−2sin3(u)
=2sin(u)−2sin3(u)
=sin(u)−2sin3(u)+2sin(u)−2sin3(u)
Simplify sin(u)−2sin3(u)+2sin(u)−2sin3(u):−4sin3(u)+3sin(u)
sin(u)−2sin3(u)+2sin(u)−2sin3(u)
Group like terms=−2sin3(u)−2sin3(u)+sin(u)+2sin(u)
Add similar elements: −2sin3(u)−2sin3(u)=−4sin3(u)=−4sin3(u)+sin(u)+2sin(u)
Add similar elements: 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)
Simplify=4sin(u)−4sin3(u)−2cos(u)sin(u)
4sin(u)−4sin3(u)−2cos(u)sin(u)=0
Factor 4sin(u)−4sin3(u)−2cos(u)sin(u):2sin(u)(2−2sin2(u)−cos(u))
4sin(u)−4sin3(u)−2cos(u)sin(u)
Apply exponent rule: ab+c=abacsin3(u)=sin(u)sin2(u)=4sin(u)−4sin(u)sin2(u)−2sin(u)cos(u)
Rewrite −4 as 2⋅2Rewrite 4 as 2⋅2=2⋅2sin(u)+2⋅2sin(u)sin2(u)−2sin(u)cos(u)
Factor out common term 2sin(u)=2sin(u)(2−2sin2(u)−cos(u))
2sin(u)(2−2sin2(u)−cos(u))=0
Solving each part separatelysin(u)=0or2−2sin2(u)−cos(u)=0
sin(u)=0:u=2πn,u=π+2πn
sin(u)=0
General solutions for sin(u)=0
sin(x) periodicity table with 2πn cycle:
x06π​4π​3π​2π​32π​43π​65π​​sin(x)021​22​​23​​123​​22​​21​​xπ67π​45π​34π​23π​35π​47π​611π​​sin(x)0−21​−22​​−23​​−1−23​​−22​​−21​​​
u=0+2πn,u=π+2πn
u=0+2πn,u=π+2πn
Solve 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
Rewrite using trig identities
2−cos(u)−2sin2(u)
Use the Pythagorean identity: cos2(x)+sin2(x)=1sin2(x)=1−cos2(x)=2−cos(u)−2(1−cos2(u))
Simplify 2−cos(u)−2(1−cos2(u)):2cos2(u)−cos(u)
2−cos(u)−2(1−cos2(u))
Expand −2(1−cos2(u)):−2+2cos2(u)
−2(1−cos2(u))
Apply the distributive law: a(b−c)=ab−aca=−2,b=1,c=cos2(u)=−2⋅1−(−2)cos2(u)
Apply minus-plus rules−(−a)=a=−2⋅1+2cos2(u)
Multiply the numbers: 2⋅1=2=−2+2cos2(u)
=2−cos(u)−2+2cos2(u)
Simplify 2−cos(u)−2+2cos2(u):2cos2(u)−cos(u)
2−cos(u)−2+2cos2(u)
Group like terms=−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
Solve by substitution
−cos(u)+2cos2(u)=0
Let: cos(u)=u−u+2u2=0
−u+2u2=0:u=21​,u=0
−u+2u2=0
Write in the standard form ax2+bx+c=02u2−u=0
Solve with the quadratic formula
2u2−u=0
Quadratic Equation Formula:
For 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
Apply exponent rule: (−a)n=an,if n is even(−1)2=12=12
Apply rule 1a=1=1
4⋅2⋅0=0
4⋅2⋅0
Apply rule 0⋅a=0=0
=1−0​
Subtract the numbers: 1−0=1=1​
Apply rule 1​=1=1
u1,2​=2⋅2−(−1)±1​
Separate the solutionsu1​=2⋅2−(−1)+1​,u2​=2⋅2−(−1)−1​
u=2⋅2−(−1)+1​:21​
2⋅2−(−1)+1​
Apply rule −(−a)=a=2⋅21+1​
Add the numbers: 1+1=2=2⋅22​
Multiply the numbers: 2⋅2=4=42​
Cancel the common factor: 2=21​
u=2⋅2−(−1)−1​:0
2⋅2−(−1)−1​
Apply rule −(−a)=a=2⋅21−1​
Subtract the numbers: 1−1=0=2⋅20​
Multiply the numbers: 2⋅2=4=40​
Apply rule a0​=0,a=0=0
The solutions to the quadratic equation are:u=21​,u=0
Substitute back u=cos(u)cos(u)=21​,cos(u)=0
cos(u)=21​,cos(u)=0
cos(u)=21​:u=3π​+2πn,u=35π​+2πn
cos(u)=21​
General solutions for cos(u)=21​
cos(x) periodicity table with 2πn cycle:
x06π​4π​3π​2π​32π​43π​65π​​cos(x)123​​22​​21​0−21​−22​​−23​​​xπ67π​45π​34π​23π​35π​47π​611π​​cos(x)−1−23​​−22​​−21​021​22​​23​​​​
u=3π​+2πn,u=35π​+2πn
u=3π​+2πn,u=35π​+2πn
cos(u)=0:u=2π​+2πn,u=23π​+2πn
cos(u)=0
General solutions for cos(u)=0
cos(x) periodicity table with 2πn cycle:
x06π​4π​3π​2π​32π​43π​65π​​cos(x)123​​22​​21​0−21​−22​​−23​​​xπ67π​45π​34π​23π​35π​47π​611π​​cos(x)−1−23​​−22​​−21​021​22​​23​​​​
u=2π​+2πn,u=23π​+2πn
u=2π​+2πn,u=23π​+2πn
Combine all the solutionsu=3π​+2πn,u=35π​+2πn,u=2π​+2πn,u=23π​+2πn
Combine all the solutionsu=2πn,u=π+2πn,u=3π​+2πn,u=35π​+2πn,u=2π​+2πn,u=23π​+2πn
Substitute back u=2x
2x=2πn:x=πn
2x=2πn
Divide both sides by 2
2x=2πn
Divide both sides by 222x​=22πn​
Simplifyx=πn
x=πn
2x=π+2πn:x=2π+2πn​
2x=π+2πn
Divide both sides by 2
2x=π+2πn
Divide both sides by 222x​=2π​+22πn​
Simplify
22x​=2π​+22πn​
Simplify 22x​:x
22x​
Divide the numbers: 22​=1=x
Simplify 2π​+22πn​:2π+2πn​
2π​+22πn​
Apply rule ca​±cb​=ca±b​=2π+2πn​
x=2π+2πn​
x=2π+2πn​
x=2π+2πn​
2x=3π​+2πn:x=6π+6πn​
2x=3π​+2πn
Divide both sides by 2
2x=3π​+2πn
Divide both sides by 222x​=23π​​+22πn​
Simplify
22x​=23π​​+22πn​
Simplify 22x​:x
22x​
Divide the numbers: 22​=1=x
Simplify 23π​​+22πn​:6π+6πn​
23π​​+22πn​
Apply rule ca​±cb​=ca±b​=23π​+2πn​
Join 3π​+2πn:3π+6πn​
3π​+2πn
Convert element to fraction: 2πn=32πn3​=3π​+32πn⋅3​
Since the denominators are equal, combine the fractions: ca​±cb​=ca±b​=3π+2πn⋅3​
Multiply the numbers: 2⋅3=6=3π+6πn​
=23π+6πn​​
Apply the fraction rule: acb​​=c⋅ab​=3⋅2π+6πn​
Multiply the numbers: 3⋅2=6=6π+6πn​
x=6π+6πn​
x=6π+6πn​
x=6π+6πn​
2x=35π​+2πn:x=65π+6πn​
2x=35π​+2πn
Divide both sides by 2
2x=35π​+2πn
Divide both sides by 222x​=235π​​+22πn​
Simplify
22x​=235π​​+22πn​
Simplify 22x​:x
22x​
Divide the numbers: 22​=1=x
Simplify 235π​​+22πn​:65π+6πn​
235π​​+22πn​
Apply rule ca​±cb​=ca±b​=235π​+2πn​
Join 35π​+2πn:35π+6πn​
35π​+2πn
Convert element to fraction: 2πn=32πn3​=35π​+32πn⋅3​
Since the denominators are equal, combine the fractions: ca​±cb​=ca±b​=35π+2πn⋅3​
Multiply the numbers: 2⋅3=6=35π+6πn​
=235π+6πn​​
Apply the fraction rule: acb​​=c⋅ab​=3⋅25π+6πn​
Multiply the numbers: 3⋅2=6=65π+6πn​
x=65π+6πn​
x=65π+6πn​
x=65π+6πn​
2x=2π​+2πn:x=4π+4πn​
2x=2π​+2πn
Divide both sides by 2
2x=2π​+2πn
Divide both sides by 222x​=22π​​+22πn​
Simplify
22x​=22π​​+22πn​
Simplify 22x​:x
22x​
Divide the numbers: 22​=1=x
Simplify 22π​​+22πn​:4π+4πn​
22π​​+22πn​
Apply rule ca​±cb​=ca±b​=22π​+2πn​
Join 2π​+2πn:2π+4πn​
2π​+2πn
Convert element to fraction: 2πn=22πn2​=2π​+22πn⋅2​
Since the denominators are equal, combine the fractions: ca​±cb​=ca±b​=2π+2πn⋅2​
Multiply the numbers: 2⋅2=4=2π+4πn​
=22π+4πn​​
Apply the fraction rule: acb​​=c⋅ab​=2⋅2π+4πn​
Multiply the numbers: 2⋅2=4=4π+4πn​
x=4π+4πn​
x=4π+4πn​
x=4π+4πn​
2x=23π​+2πn:x=43π+4πn​
2x=23π​+2πn
Divide both sides by 2
2x=23π​+2πn
Divide both sides by 222x​=223π​​+22πn​
Simplify
22x​=223π​​+22πn​
Simplify 22x​:x
22x​
Divide the numbers: 22​=1=x
Simplify 223π​​+22πn​:43π+4πn​
223π​​+22πn​
Apply rule ca​±cb​=ca±b​=223π​+2πn​
Join 23π​+2πn:23π+4πn​
23π​+2πn
Convert element to fraction: 2πn=22πn2​=23π​+22πn⋅2​
Since the denominators are equal, combine the fractions: ca​±cb​=ca±b​=23π+2πn⋅2​
Multiply the numbers: 2⋅2=4=23π+4πn​
=223π+4πn​​
Apply the fraction rule: acb​​=c⋅ab​=2⋅23π+4πn​
Multiply the numbers: 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​

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