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Popular Trigonometry >

2sin(4x)=sin(2x)

  • Pre Algebra
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Solution

2sin(4x)=sin(2x)

Solution

x=πn,x=2π+2πn​,x=21.31811…+2πn​,x=22π−1.31811…+2πn​
+1
Degrees
x=0∘+180∘n,x=90∘+180∘n,x=37.76124…∘+180∘n,x=142.23875…∘+180∘n
Solution steps
2sin(4x)=sin(2x)
Subtract sin(2x) from both sides2sin(4x)−sin(2x)=0
Let: u=2x2sin(2u)−sin(u)=0
Rewrite using trig identities
−sin(u)+2sin(2u)
Use the Double Angle identity: sin(2x)=2sin(x)cos(x)=−sin(u)+2⋅2sin(u)cos(u)
Simplify=−sin(u)+4sin(u)cos(u)
−sin(u)+4cos(u)sin(u)=0
Factor −sin(u)+4cos(u)sin(u):sin(u)(4cos(u)−1)
−sin(u)+4cos(u)sin(u)
Factor out common term sin(u)=sin(u)(−1+4cos(u))
sin(u)(4cos(u)−1)=0
Solving each part separatelysin(u)=0or4cos(u)−1=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
4cos(u)−1=0:u=arccos(41​)+2πn,u=2π−arccos(41​)+2πn
4cos(u)−1=0
Move 1to the right side
4cos(u)−1=0
Add 1 to both sides4cos(u)−1+1=0+1
Simplify4cos(u)=1
4cos(u)=1
Divide both sides by 4
4cos(u)=1
Divide both sides by 444cos(u)​=41​
Simplifycos(u)=41​
cos(u)=41​
Apply trig inverse properties
cos(u)=41​
General solutions for cos(u)=41​cos(x)=a⇒x=arccos(a)+2πn,x=2π−arccos(a)+2πnu=arccos(41​)+2πn,u=2π−arccos(41​)+2πn
u=arccos(41​)+2πn,u=2π−arccos(41​)+2πn
Combine all the solutionsu=2πn,u=π+2πn,u=arccos(41​)+2πn,u=2π−arccos(41​)+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=arccos(41​)+2πn:x=2arccos(41​)+2πn​
2x=arccos(41​)+2πn
Divide both sides by 2
2x=arccos(41​)+2πn
Divide both sides by 222x​=2arccos(41​)​+22πn​
Simplify
22x​=2arccos(41​)​+22πn​
Simplify 22x​:x
22x​
Divide the numbers: 22​=1=x
Simplify 2arccos(41​)​+22πn​:2arccos(41​)+2πn​
2arccos(41​)​+22πn​
Apply rule ca​±cb​=ca±b​=2arccos(41​)+2πn​
x=2arccos(41​)+2πn​
x=2arccos(41​)+2πn​
x=2arccos(41​)+2πn​
2x=2π−arccos(41​)+2πn:x=22π−arccos(41​)+2πn​
2x=2π−arccos(41​)+2πn
Divide both sides by 2
2x=2π−arccos(41​)+2πn
Divide both sides by 222x​=22π​−2arccos(41​)​+22πn​
Simplify
22x​=22π​−2arccos(41​)​+22πn​
Simplify 22x​:x
22x​
Divide the numbers: 22​=1=x
Simplify 22π​−2arccos(41​)​+22πn​:22π−arccos(41​)+2πn​
22π​−2arccos(41​)​+22πn​
Apply rule ca​±cb​=ca±b​=22π−arccos(41​)+2πn​
x=22π−arccos(41​)+2πn​
x=22π−arccos(41​)+2πn​
x=22π−arccos(41​)+2πn​
x=πn,x=2π+2πn​,x=2arccos(41​)+2πn​,x=22π−arccos(41​)+2πn​
Show solutions in decimal formx=πn,x=2π+2πn​,x=21.31811…+2πn​,x=22π−1.31811…+2πn​

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