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Popular Trigonometry >

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

  • Pre Algebra
  • Algebra
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Solution

2−2cos2(2x​)=2sin2(x)

Solution

x=4πn,x=2π+4πn,x=34π​+4πn,x=38π​+4πn,x=32π+8πn​,x=2π+38πn​,x=8πn,x=4π+8πn
+1
Degrees
x=0∘+720∘n,x=360∘+720∘n,x=240∘+720∘n,x=480∘+720∘n,x=120∘+480∘n,x=360∘+480∘n,x=0∘+1440∘n,x=720∘+1440∘n
Solution steps
2−2cos2(2x​)=2sin2(x)
Subtract 2sin2(x) from both sides2−2cos2(2x​)−2sin2(x)=0
Let: u=2x​2−2cos2(u)−2sin2(2u)=0
Rewrite using trig identities
2−2cos2(u)−2sin2(2u)
Use the Pythagorean identity: cos2(x)+sin2(x)=1cos2(x)=1−sin2(x)=2−2(1−sin2(u))−2sin2(2u)
Simplify 2−2(1−sin2(u))−2sin2(2u):2sin2(u)−2sin2(2u)
2−2(1−sin2(u))−2sin2(2u)
Expand −2(1−sin2(u)):−2+2sin2(u)
−2(1−sin2(u))
Apply the distributive law: a(b−c)=ab−aca=−2,b=1,c=sin2(u)=−2⋅1−(−2)sin2(u)
Apply minus-plus rules−(−a)=a=−2⋅1+2sin2(u)
Multiply the numbers: 2⋅1=2=−2+2sin2(u)
=2−2+2sin2(u)−2sin2(2u)
2−2=0=2sin2(u)−2sin2(2u)
=2sin2(u)−2sin2(2u)
−2sin2(2u)+2sin2(u)=0
Factor −2sin2(2u)+2sin2(u):2(sin(u)+sin(2u))(sin(u)−sin(2u))
−2sin2(2u)+2sin2(u)
Factor out common term 2=2(−sin2(2u)+sin2(u))
Factor sin2(u)−sin2(2u):(sin(u)+sin(2u))(sin(u)−sin(2u))
sin2(u)−sin2(2u)
Apply Difference of Two Squares Formula: x2−y2=(x+y)(x−y)sin2(u)−sin2(2u)=(sin(u)+sin(2u))(sin(u)−sin(2u))=(sin(u)+sin(2u))(sin(u)−sin(2u))
=2(sin(u)+sin(2u))(sin(u)−sin(2u))
2(sin(u)+sin(2u))(sin(u)−sin(2u))=0
Solving each part separatelysin(u)+sin(2u)=0orsin(u)−sin(2u)=0
sin(u)+sin(2u)=0:u=2πn,u=π+2πn,u=32π​+2πn,u=34π​+2πn
sin(u)+sin(2u)=0
Rewrite using trig identities
sin(2u)+sin(u)
Use the Double Angle identity: sin(2x)=2sin(x)cos(x)=2sin(u)cos(u)+sin(u)
sin(u)+2cos(u)sin(u)=0
Factor sin(u)+2cos(u)sin(u):sin(u)(2cos(u)+1)
sin(u)+2cos(u)sin(u)
Factor out common term sin(u)=sin(u)(1+2cos(u))
sin(u)(2cos(u)+1)=0
Solving each part separatelysin(u)=0or2cos(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
2cos(u)+1=0:u=32π​+2πn,u=34π​+2πn
2cos(u)+1=0
Move 1to the right side
2cos(u)+1=0
Subtract 1 from both sides2cos(u)+1−1=0−1
Simplify2cos(u)=−1
2cos(u)=−1
Divide both sides by 2
2cos(u)=−1
Divide both sides by 222cos(u)​=2−1​
Simplifycos(u)=−21​
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=32π​+2πn,u=34π​+2πn
u=32π​+2πn,u=34π​+2πn
Combine all the solutionsu=2πn,u=π+2πn,u=32π​+2πn,u=34π​+2πn
sin(u)−sin(2u)=0:u=3π​+34πn​,u=π+34πn​,u=4πn,u=2π+4πn
sin(u)−sin(2u)=0
Rewrite using trig identities
−sin(2u)+sin(u)
Use the Sum to Product identity: sin(s)−sin(t)=2sin(2s−t​)cos(2s+t​)=2sin(2u−2u​)cos(2u+2u​)
Simplify 2sin(2u−2u​)cos(2u+2u​):−2cos(23u​)sin(2u​)
2sin(2u−2u​)cos(2u+2u​)
2u−2u​=−2u​
2u−2u​
Add similar elements: u−2u=−u=2−u​
Apply the fraction rule: b−a​=−ba​=−2u​
=2sin(−2u​)cos(2u+2u​)
Use the negative angle identity: sin(−x)=−sin(x)=2cos(2u+2u​)(−sin(2u​))
Remove parentheses: (−a)=−a=−2cos(2u+2u​)sin(2u​)
Add similar elements: u+2u=3u=−2cos(23u​)sin(2u​)
=−2cos(23u​)sin(2u​)
−2cos(23u​)sin(2u​)=0
Solving each part separatelycos(23u​)=0orsin(2u​)=0
cos(23u​)=0:u=3π​+34πn​,u=π+34πn​
cos(23u​)=0
General solutions for cos(23u​)=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​​​​
23u​=2π​+2πn,23u​=23π​+2πn
23u​=2π​+2πn,23u​=23π​+2πn
Solve 23u​=2π​+2πn:u=3π​+34πn​
23u​=2π​+2πn
Multiply both sides by 2
23u​=2π​+2πn
Multiply both sides by 222⋅3u​=2⋅2π​+2⋅2πn
Simplify
22⋅3u​=2⋅2π​+2⋅2πn
Simplify 22⋅3u​:3u
22⋅3u​
Multiply the numbers: 2⋅3=6=26u​
Divide the numbers: 26​=3=3u
Simplify 2⋅2π​+2⋅2πn:π+4πn
2⋅2π​+2⋅2πn
2⋅2π​=π
2⋅2π​
Multiply fractions: a⋅cb​=ca⋅b​=2π2​
Cancel the common factor: 2=π
2⋅2πn=4πn
2⋅2πn
Multiply the numbers: 2⋅2=4=4πn
=π+4πn
3u=π+4πn
3u=π+4πn
3u=π+4πn
Divide both sides by 3
3u=π+4πn
Divide both sides by 333u​=3π​+34πn​
Simplifyu=3π​+34πn​
u=3π​+34πn​
Solve 23u​=23π​+2πn:u=π+34πn​
23u​=23π​+2πn
Multiply both sides by 2
23u​=23π​+2πn
Multiply both sides by 222⋅3u​=2⋅23π​+2⋅2πn
Simplify
22⋅3u​=2⋅23π​+2⋅2πn
Simplify 22⋅3u​:3u
22⋅3u​
Multiply the numbers: 2⋅3=6=26u​
Divide the numbers: 26​=3=3u
Simplify 2⋅23π​+2⋅2πn:3π+4πn
2⋅23π​+2⋅2πn
2⋅23π​=3π
2⋅23π​
Multiply fractions: a⋅cb​=ca⋅b​=23π2​
Cancel the common factor: 2=3π
2⋅2πn=4πn
2⋅2πn
Multiply the numbers: 2⋅2=4=4πn
=3π+4πn
3u=3π+4πn
3u=3π+4πn
3u=3π+4πn
Divide both sides by 3
3u=3π+4πn
Divide both sides by 333u​=33π​+34πn​
Simplifyu=π+34πn​
u=π+34πn​
u=3π​+34πn​,u=π+34πn​
sin(2u​)=0:u=4πn,u=2π+4πn
sin(2u​)=0
General solutions for sin(2u​)=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​​​
2u​=0+2πn,2u​=π+2πn
2u​=0+2πn,2u​=π+2πn
Solve 2u​=0+2πn:u=4πn
2u​=0+2πn
0+2πn=2πn2u​=2πn
Multiply both sides by 2
2u​=2πn
Multiply both sides by 222u​=2⋅2πn
Simplifyu=4πn
u=4πn
Solve 2u​=π+2πn:u=2π+4πn
2u​=π+2πn
Multiply both sides by 2
2u​=π+2πn
Multiply both sides by 222u​=2π+2⋅2πn
Simplifyu=2π+4πn
u=2π+4πn
u=4πn,u=2π+4πn
Combine all the solutionsu=3π​+34πn​,u=π+34πn​,u=4πn,u=2π+4πn
Combine all the solutionsu=2πn,u=π+2πn,u=32π​+2πn,u=34π​+2πn,u=3π​+34πn​,u=π+34πn​,u=4πn,u=2π+4πn
Substitute back u=2x​
2x​=2πn:x=4πn
2x​=2πn
Multiply both sides by 2
2x​=2πn
Multiply both sides by 222x​=2⋅2πn
Simplifyx=4πn
x=4πn
2x​=π+2πn:x=2π+4πn
2x​=π+2πn
Multiply both sides by 2
2x​=π+2πn
Multiply both sides by 222x​=2π+2⋅2πn
Simplifyx=2π+4πn
x=2π+4πn
2x​=32π​+2πn:x=34π​+4πn
2x​=32π​+2πn
Multiply both sides by 2
2x​=32π​+2πn
Multiply both sides by 222x​=2⋅32π​+2⋅2πn
Simplify
22x​=2⋅32π​+2⋅2πn
Simplify 22x​:x
22x​
Divide the numbers: 22​=1=x
Simplify 2⋅32π​+2⋅2πn:34π​+4πn
2⋅32π​+2⋅2πn
2⋅32π​=34π​
2⋅32π​
Multiply fractions: a⋅cb​=ca⋅b​=32π2​
Multiply the numbers: 2⋅2=4=34π​
2⋅2πn=4πn
2⋅2πn
Multiply the numbers: 2⋅2=4=4πn
=34π​+4πn
x=34π​+4πn
x=34π​+4πn
x=34π​+4πn
2x​=34π​+2πn:x=38π​+4πn
2x​=34π​+2πn
Multiply both sides by 2
2x​=34π​+2πn
Multiply both sides by 222x​=2⋅34π​+2⋅2πn
Simplify
22x​=2⋅34π​+2⋅2πn
Simplify 22x​:x
22x​
Divide the numbers: 22​=1=x
Simplify 2⋅34π​+2⋅2πn:38π​+4πn
2⋅34π​+2⋅2πn
2⋅34π​=38π​
2⋅34π​
Multiply fractions: a⋅cb​=ca⋅b​=34π2​
Multiply the numbers: 4⋅2=8=38π​
2⋅2πn=4πn
2⋅2πn
Multiply the numbers: 2⋅2=4=4πn
=38π​+4πn
x=38π​+4πn
x=38π​+4πn
x=38π​+4πn
2x​=3π​+34πn​:x=32π+8πn​
2x​=3π​+34πn​
Multiply both sides by 2
2x​=3π​+34πn​
Multiply both sides by 222x​=2⋅3π​+2⋅34πn​
Simplify
22x​=2⋅3π​+2⋅34πn​
Simplify 22x​:x
22x​
Divide the numbers: 22​=1=x
Simplify 2⋅3π​+2⋅34πn​:32π+8πn​
2⋅3π​+2⋅34πn​
2⋅3π​=32π​
2⋅3π​
Multiply fractions: a⋅cb​=ca⋅b​=3π2​
2⋅34πn​=38πn​
2⋅34πn​
Multiply fractions: a⋅cb​=ca⋅b​=34πn⋅2​
Multiply the numbers: 4⋅2=8=38πn​
=32π​+38πn​
Apply rule ca​±cb​=ca±b​=32π+8πn​
x=32π+8πn​
x=32π+8πn​
x=32π+8πn​
2x​=π+34πn​:x=2π+38πn​
2x​=π+34πn​
Multiply both sides by 2
2x​=π+34πn​
Multiply both sides by 222x​=2π+2⋅34πn​
Simplify
22x​=2π+2⋅34πn​
Simplify 22x​:x
22x​
Divide the numbers: 22​=1=x
Simplify 2π+2⋅34πn​:2π+38πn​
2π+2⋅34πn​
2⋅34πn​=38πn​
2⋅34πn​
Multiply fractions: a⋅cb​=ca⋅b​=34πn⋅2​
Multiply the numbers: 4⋅2=8=38πn​
=2π+38πn​
x=2π+38πn​
x=2π+38πn​
x=2π+38πn​
2x​=4πn:x=8πn
2x​=4πn
Multiply both sides by 2
2x​=4πn
Multiply both sides by 222x​=2⋅4πn
Simplifyx=8πn
x=8πn
2x​=2π+4πn:x=4π+8πn
2x​=2π+4πn
Multiply both sides by 2
2x​=2π+4πn
Multiply both sides by 222x​=2⋅2π+2⋅4πn
Simplifyx=4π+8πn
x=4π+8πn
x=4πn,x=2π+4πn,x=34π​+4πn,x=38π​+4πn,x=32π+8πn​,x=2π+38πn​,x=8πn,x=4π+8πn
Merge Overlapping Intervalsx=4πn,x=2π+4πn,x=34π​+4πn,x=38π​+4πn,x=32π+8πn​,x=2π+38πn​,x=8πn,x=4π+8πn

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