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Popular Trigonometry >

1-2cos^2(8x)=sin(4x)

  • Pre Algebra
  • Algebra
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Solution

1−2cos2(8x)=sin(4x)

Solution

x=83π+4πn​,x=24π+12πn​,x=245π+12πn​,x=40.94247…+2πn​,x=4π−0.94247…+2πn​,x=4−0.31415…+2πn​,x=4π+0.31415…+2πn​
+1
Degrees
x=67.5∘+90∘n,x=7.5∘+90∘n,x=37.5∘+90∘n,x=13.5∘+90∘n,x=31.5∘+90∘n,x=−4.5∘+90∘n,x=49.5∘+90∘n
Solution steps
1−2cos2(8x)=sin(4x)
Subtract sin(4x) from both sides1−2cos2(8x)−sin(4x)=0
Let: u=4x1−2cos2(2u)−sin(u)=0
Rewrite using trig identities
1−sin(u)−2cos2(2u)
Use the Double Angle identity: cos(2x)=1−2sin2(x)=1−sin(u)−2(1−2sin2(u))2
Simplify 1−sin(u)−2(1−2sin2(u))2:8sin2(u)−8sin4(u)−sin(u)−1
1−sin(u)−2(1−2sin2(u))2
(1−2sin2(u))2:1−4sin2(u)+4sin4(u)
Apply Perfect Square Formula: (a−b)2=a2−2ab+b2a=1,b=2sin2(u)
=12−2⋅1⋅2sin2(u)+(2sin2(u))2
Simplify 12−2⋅1⋅2sin2(u)+(2sin2(u))2:1−4sin2(u)+4sin4(u)
12−2⋅1⋅2sin2(u)+(2sin2(u))2
Apply rule 1a=112=1=1−2⋅1⋅2sin2(u)+(2sin2(u))2
2⋅1⋅2sin2(u)=4sin2(u)
2⋅1⋅2sin2(u)
Multiply the numbers: 2⋅1⋅2=4=4sin2(u)
(2sin2(u))2=4sin4(u)
(2sin2(u))2
Apply exponent rule: (a⋅b)n=anbn=22(sin2(u))2
(sin2(u))2:sin4(u)
Apply exponent rule: (ab)c=abc=sin2⋅2(u)
Multiply the numbers: 2⋅2=4=sin4(u)
=22sin4(u)
22=4=4sin4(u)
=1−4sin2(u)+4sin4(u)
=1−4sin2(u)+4sin4(u)
=1−sin(u)−2(1−4sin2(u)+4sin4(u))
Expand −2(1−4sin2(u)+4sin4(u)):−2+8sin2(u)−8sin4(u)
−2(1−4sin2(u)+4sin4(u))
Distribute parentheses=(−2)⋅1+(−2)(−4sin2(u))+(−2)⋅4sin4(u)
Apply minus-plus rules+(−a)=−a,(−a)(−b)=ab=−2⋅1+2⋅4sin2(u)−2⋅4sin4(u)
Simplify −2⋅1+2⋅4sin2(u)−2⋅4sin4(u):−2+8sin2(u)−8sin4(u)
−2⋅1+2⋅4sin2(u)−2⋅4sin4(u)
Multiply the numbers: 2⋅1=2=−2+2⋅4sin2(u)−2⋅4sin4(u)
Multiply the numbers: 2⋅4=8=−2+8sin2(u)−8sin4(u)
=−2+8sin2(u)−8sin4(u)
=1−sin(u)−2+8sin2(u)−8sin4(u)
Simplify 1−sin(u)−2+8sin2(u)−8sin4(u):8sin2(u)−8sin4(u)−sin(u)−1
1−sin(u)−2+8sin2(u)−8sin4(u)
Group like terms=−sin(u)+8sin2(u)−8sin4(u)+1−2
Add/Subtract the numbers: 1−2=−1=8sin2(u)−8sin4(u)−sin(u)−1
=8sin2(u)−8sin4(u)−sin(u)−1
=8sin2(u)−8sin4(u)−sin(u)−1
−1−sin(u)+8sin2(u)−8sin4(u)=0
Solve by substitution
−1−sin(u)+8sin2(u)−8sin4(u)=0
Let: sin(u)=u−1−u+8u2−8u4=0
−1−u+8u2−8u4=0:u=−1,u=21​,u=41+5​​,u=41−5​​
−1−u+8u2−8u4=0
Write in the standard form an​xn+…+a1​x+a0​=0−8u4+8u2−u−1=0
Factor −8u4+8u2−u−1:−(u+1)(2u−1)(4u2−2u−1)
−8u4+8u2−u−1
Factor out common term −1=−(8u4−8u2+u+1)
Factor 8u4−8u2+u+1:(u+1)(2u−1)(4u2−2u−1)
8u4−8u2+u+1
Use the rational root theorem
a0​=1,an​=8
The dividers of a0​:1,The dividers of an​:1,2,4,8
Therefore, check the following rational numbers:±1,2,4,81​
−11​ is a root of the expression, so factor out u+1
=(u+1)u+18u4−8u2+u+1​
u+18u4−8u2+u+1​=8u3−8u2+1
u+18u4−8u2+u+1​
Divide u+18u4−8u2+u+1​:u+18u4−8u2+u+1​=8u3+u+1−8u3−8u2+u+1​
Divide the leading coefficients of the numerator 8u4−8u2+u+1
and the divisor u+1:u8u4​=8u3
Quotient=8u3
Multiply u+1 by 8u3:8u4+8u3Subtract 8u4+8u3 from 8u4−8u2+u+1 to get new remainderRemainder=−8u3−8u2+u+1
Thereforeu+18u4−8u2+u+1​=8u3+u+1−8u3−8u2+u+1​
=8u3+u+1−8u3−8u2+u+1​
Divide u+1−8u3−8u2+u+1​:u+1−8u3−8u2+u+1​=−8u2+u+1u+1​
Divide the leading coefficients of the numerator −8u3−8u2+u+1
and the divisor u+1:u−8u3​=−8u2
Quotient=−8u2
Multiply u+1 by −8u2:−8u3−8u2Subtract −8u3−8u2 from −8u3−8u2+u+1 to get new remainderRemainder=u+1
Thereforeu+1−8u3−8u2+u+1​=−8u2+u+1u+1​
=8u3−8u2+u+1u+1​
Divide u+1u+1​:u+1u+1​=1
Divide the leading coefficients of the numerator u+1
and the divisor u+1:uu​=1
Quotient=1
Multiply u+1 by 1:u+1Subtract u+1 from u+1 to get new remainderRemainder=0
Thereforeu+1u+1​=1
=8u3−8u2+1
=8u3−8u2+1
Factor 8u3−8u2+1:(2u−1)(4u2−2u−1)
8u3−8u2+1
Use the rational root theorem
a0​=1,an​=8
The dividers of a0​:1,The dividers of an​:1,2,4,8
Therefore, check the following rational numbers:±1,2,4,81​
21​ is a root of the expression, so factor out 2u−1
=(2u−1)2u−18u3−8u2+1​
2u−18u3−8u2+1​=4u2−2u−1
2u−18u3−8u2+1​
Divide 2u−18u3−8u2+1​:2u−18u3−8u2+1​=4u2+2u−1−4u2+1​
Divide the leading coefficients of the numerator 8u3−8u2+1
and the divisor 2u−1:2u8u3​=4u2
Quotient=4u2
Multiply 2u−1 by 4u2:8u3−4u2Subtract 8u3−4u2 from 8u3−8u2+1 to get new remainderRemainder=−4u2+1
Therefore2u−18u3−8u2+1​=4u2+2u−1−4u2+1​
=4u2+2u−1−4u2+1​
Divide 2u−1−4u2+1​:2u−1−4u2+1​=−2u+2u−1−2u+1​
Divide the leading coefficients of the numerator −4u2+1
and the divisor 2u−1:2u−4u2​=−2u
Quotient=−2u
Multiply 2u−1 by −2u:−4u2+2uSubtract −4u2+2u from −4u2+1 to get new remainderRemainder=−2u+1
Therefore2u−1−4u2+1​=−2u+2u−1−2u+1​
=4u2−2u+2u−1−2u+1​
Divide 2u−1−2u+1​:2u−1−2u+1​=−1
Divide the leading coefficients of the numerator −2u+1
and the divisor 2u−1:2u−2u​=−1
Quotient=−1
Multiply 2u−1 by −1:−2u+1Subtract −2u+1 from −2u+1 to get new remainderRemainder=0
Therefore2u−1−2u+1​=−1
=4u2−2u−1
=4u2−2u−1
=(2u−1)(4u2−2u−1)
=(u+1)(2u−1)(4u2−2u−1)
=−(u+1)(2u−1)(4u2−2u−1)
−(u+1)(2u−1)(4u2−2u−1)=0
Using the Zero Factor Principle: If ab=0then a=0or b=0u+1=0or2u−1=0or4u2−2u−1=0
Solve u+1=0:u=−1
u+1=0
Move 1to the right side
u+1=0
Subtract 1 from both sidesu+1−1=0−1
Simplifyu=−1
u=−1
Solve 2u−1=0:u=21​
2u−1=0
Move 1to the right side
2u−1=0
Add 1 to both sides2u−1+1=0+1
Simplify2u=1
2u=1
Divide both sides by 2
2u=1
Divide both sides by 222u​=21​
Simplifyu=21​
u=21​
Solve 4u2−2u−1=0:u=41+5​​,u=41−5​​
4u2−2u−1=0
Solve with the quadratic formula
4u2−2u−1=0
Quadratic Equation Formula:
For a=4,b=−2,c=−1u1,2​=2⋅4−(−2)±(−2)2−4⋅4(−1)​​
u1,2​=2⋅4−(−2)±(−2)2−4⋅4(−1)​​
(−2)2−4⋅4(−1)​=25​
(−2)2−4⋅4(−1)​
Apply rule −(−a)=a=(−2)2+4⋅4⋅1​
Apply exponent rule: (−a)n=an,if n is even(−2)2=22=22+4⋅4⋅1​
Multiply the numbers: 4⋅4⋅1=16=22+16​
22=4=4+16​
Add the numbers: 4+16=20=20​
Prime factorization of 20:22⋅5
20
20divides by 220=10⋅2=2⋅10
10divides by 210=5⋅2=2⋅2⋅5
2,5 are all prime numbers, therefore no further factorization is possible=2⋅2⋅5
=22⋅5
=22⋅5​
Apply radical rule: =5​22​
Apply radical rule: 22​=2=25​
u1,2​=2⋅4−(−2)±25​​
Separate the solutionsu1​=2⋅4−(−2)+25​​,u2​=2⋅4−(−2)−25​​
u=2⋅4−(−2)+25​​:41+5​​
2⋅4−(−2)+25​​
Apply rule −(−a)=a=2⋅42+25​​
Multiply the numbers: 2⋅4=8=82+25​​
Factor 2+25​:2(1+5​)
2+25​
Rewrite as=2⋅1+25​
Factor out common term 2=2(1+5​)
=82(1+5​)​
Cancel the common factor: 2=41+5​​
u=2⋅4−(−2)−25​​:41−5​​
2⋅4−(−2)−25​​
Apply rule −(−a)=a=2⋅42−25​​
Multiply the numbers: 2⋅4=8=82−25​​
Factor 2−25​:2(1−5​)
2−25​
Rewrite as=2⋅1−25​
Factor out common term 2=2(1−5​)
=82(1−5​)​
Cancel the common factor: 2=41−5​​
The solutions to the quadratic equation are:u=41+5​​,u=41−5​​
The solutions areu=−1,u=21​,u=41+5​​,u=41−5​​
Substitute back u=sin(u)sin(u)=−1,sin(u)=21​,sin(u)=41+5​​,sin(u)=41−5​​
sin(u)=−1,sin(u)=21​,sin(u)=41+5​​,sin(u)=41−5​​
sin(u)=−1:u=23π​+2πn
sin(u)=−1
General solutions for sin(u)=−1
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=23π​+2πn
u=23π​+2πn
sin(u)=21​:u=6π​+2πn,u=65π​+2πn
sin(u)=21​
General solutions for sin(u)=21​
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=6π​+2πn,u=65π​+2πn
u=6π​+2πn,u=65π​+2πn
sin(u)=41+5​​:u=arcsin(41+5​​)+2πn,u=π−arcsin(41+5​​)+2πn
sin(u)=41+5​​
Apply trig inverse properties
sin(u)=41+5​​
General solutions for sin(u)=41+5​​sin(x)=a⇒x=arcsin(a)+2πn,x=π−arcsin(a)+2πnu=arcsin(41+5​​)+2πn,u=π−arcsin(41+5​​)+2πn
u=arcsin(41+5​​)+2πn,u=π−arcsin(41+5​​)+2πn
sin(u)=41−5​​:u=arcsin(41−5​​)+2πn,u=π+arcsin(−41−5​​)+2πn
sin(u)=41−5​​
Apply trig inverse properties
sin(u)=41−5​​
General solutions for sin(u)=41−5​​sin(x)=−a⇒x=arcsin(−a)+2πn,x=π+arcsin(a)+2πnu=arcsin(41−5​​)+2πn,u=π+arcsin(−41−5​​)+2πn
u=arcsin(41−5​​)+2πn,u=π+arcsin(−41−5​​)+2πn
Combine all the solutionsu=23π​+2πn,u=6π​+2πn,u=65π​+2πn,u=arcsin(41+5​​)+2πn,u=π−arcsin(41+5​​)+2πn,u=arcsin(41−5​​)+2πn,u=π+arcsin(−41−5​​)+2πn
Substitute back u=4x
4x=23π​+2πn:x=83π+4πn​
4x=23π​+2πn
Divide both sides by 4
4x=23π​+2πn
Divide both sides by 444x​=423π​​+42πn​
Simplify
44x​=423π​​+42πn​
Simplify 44x​:x
44x​
Divide the numbers: 44​=1=x
Simplify 423π​​+42πn​:83π+4πn​
423π​​+42πn​
Apply rule ca​±cb​=ca±b​=423π​+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​
=423π+4πn​​
Apply the fraction rule: acb​​=c⋅ab​=2⋅43π+4πn​
Multiply the numbers: 2⋅4=8=83π+4πn​
x=83π+4πn​
x=83π+4πn​
x=83π+4πn​
4x=6π​+2πn:x=24π+12πn​
4x=6π​+2πn
Divide both sides by 4
4x=6π​+2πn
Divide both sides by 444x​=46π​​+42πn​
Simplify
44x​=46π​​+42πn​
Simplify 44x​:x
44x​
Divide the numbers: 44​=1=x
Simplify 46π​​+42πn​:24π+12πn​
46π​​+42πn​
Apply rule ca​±cb​=ca±b​=46π​+2πn​
Join 6π​+2πn:6π+12πn​
6π​+2πn
Convert element to fraction: 2πn=62πn6​=6π​+62πn⋅6​
Since the denominators are equal, combine the fractions: ca​±cb​=ca±b​=6π+2πn⋅6​
Multiply the numbers: 2⋅6=12=6π+12πn​
=46π+12πn​​
Apply the fraction rule: acb​​=c⋅ab​=6⋅4π+12πn​
Multiply the numbers: 6⋅4=24=24π+12πn​
x=24π+12πn​
x=24π+12πn​
x=24π+12πn​
4x=65π​+2πn:x=245π+12πn​
4x=65π​+2πn
Divide both sides by 4
4x=65π​+2πn
Divide both sides by 444x​=465π​​+42πn​
Simplify
44x​=465π​​+42πn​
Simplify 44x​:x
44x​
Divide the numbers: 44​=1=x
Simplify 465π​​+42πn​:245π+12πn​
465π​​+42πn​
Apply rule ca​±cb​=ca±b​=465π​+2πn​
Join 65π​+2πn:65π+12πn​
65π​+2πn
Convert element to fraction: 2πn=62πn6​=65π​+62πn⋅6​
Since the denominators are equal, combine the fractions: ca​±cb​=ca±b​=65π+2πn⋅6​
Multiply the numbers: 2⋅6=12=65π+12πn​
=465π+12πn​​
Apply the fraction rule: acb​​=c⋅ab​=6⋅45π+12πn​
Multiply the numbers: 6⋅4=24=245π+12πn​
x=245π+12πn​
x=245π+12πn​
x=245π+12πn​
4x=arcsin(41+5​​)+2πn:x=4arcsin(41+5​​)+2πn​
4x=arcsin(41+5​​)+2πn
Divide both sides by 4
4x=arcsin(41+5​​)+2πn
Divide both sides by 444x​=4arcsin(41+5​​)​+42πn​
Simplify
44x​=4arcsin(41+5​​)​+42πn​
Simplify 44x​:x
44x​
Divide the numbers: 44​=1=x
Simplify 4arcsin(41+5​​)​+42πn​:4arcsin(41+5​​)+2πn​
4arcsin(41+5​​)​+42πn​
Apply rule ca​±cb​=ca±b​=4arcsin(41+5​​)+2πn​
x=4arcsin(41+5​​)+2πn​
x=4arcsin(41+5​​)+2πn​
x=4arcsin(41+5​​)+2πn​
4x=π−arcsin(41+5​​)+2πn:x=4π−arcsin(41+5​​)+2πn​
4x=π−arcsin(41+5​​)+2πn
Divide both sides by 4
4x=π−arcsin(41+5​​)+2πn
Divide both sides by 444x​=4π​−4arcsin(41+5​​)​+42πn​
Simplify
44x​=4π​−4arcsin(41+5​​)​+42πn​
Simplify 44x​:x
44x​
Divide the numbers: 44​=1=x
Simplify 4π​−4arcsin(41+5​​)​+42πn​:4π−arcsin(41+5​​)+2πn​
4π​−4arcsin(41+5​​)​+42πn​
Apply rule ca​±cb​=ca±b​=4π−arcsin(41+5​​)+2πn​
x=4π−arcsin(41+5​​)+2πn​
x=4π−arcsin(41+5​​)+2πn​
x=4π−arcsin(41+5​​)+2πn​
4x=arcsin(41−5​​)+2πn:x=4arcsin(41−5​​)+2πn​
4x=arcsin(41−5​​)+2πn
Divide both sides by 4
4x=arcsin(41−5​​)+2πn
Divide both sides by 444x​=4arcsin(41−5​​)​+42πn​
Simplify
44x​=4arcsin(41−5​​)​+42πn​
Simplify 44x​:x
44x​
Divide the numbers: 44​=1=x
Simplify 4arcsin(41−5​​)​+42πn​:4arcsin(41−5​​)+2πn​
4arcsin(41−5​​)​+42πn​
Apply rule ca​±cb​=ca±b​=4arcsin(41−5​​)+2πn​
x=4arcsin(41−5​​)+2πn​
x=4arcsin(41−5​​)+2πn​
x=4arcsin(41−5​​)+2πn​
4x=π+arcsin(−41−5​​)+2πn:x=4π+arcsin(45​−1​)+2πn​
4x=π+arcsin(−41−5​​)+2πn
−41−5​​=−4−(5​−1)​=45​−1​4x=π+arcsin(45​−1​)+2πn
Divide both sides by 4
4x=π+arcsin(45​−1​)+2πn
Divide both sides by 444x​=4π​+4arcsin(45​−1​)​+42πn​
Simplify
44x​=4π​+4arcsin(45​−1​)​+42πn​
Simplify 44x​:x
44x​
Divide the numbers: 44​=1=x
Simplify 4π​+4arcsin(45​−1​)​+42πn​:4π+arcsin(45​−1​)+2πn​
4π​+4arcsin(45​−1​)​+42πn​
Apply rule ca​±cb​=ca±b​=4π+arcsin(45​−1​)+2πn​
x=4π+arcsin(45​−1​)+2πn​
x=4π+arcsin(45​−1​)+2πn​
x=4π+arcsin(45​−1​)+2πn​
x=83π+4πn​,x=24π+12πn​,x=245π+12πn​,x=4arcsin(41+5​​)+2πn​,x=4π−arcsin(41+5​​)+2πn​,x=4arcsin(41−5​​)+2πn​,x=4π+arcsin(45​−1​)+2πn​
Show solutions in decimal formx=83π+4πn​,x=24π+12πn​,x=245π+12πn​,x=40.94247…+2πn​,x=4π−0.94247…+2πn​,x=4−0.31415…+2πn​,x=4π+0.31415…+2πn​

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Frequently Asked Questions (FAQ)

  • What is the general solution for 1-2cos^2(8x)=sin(4x) ?

    The general solution for 1-2cos^2(8x)=sin(4x) is
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