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3.4: Trigonometric Equations Using Factoring

Difficulty Level: At Grade Created by: CK-12
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Solving trig equations is an important process in mathematics. Quite often you'll see powers of trigonometric functions and be asked to solve for the values of the variable which make the equation true. For example, suppose you were given the trig equation

2sinxcosx=cosx

Could you solve this equation? (You might be tempted to just divide both sides by cosx, but that would be incorrect because you would lose some solutions.) Instead, you're going to have to use factoring. Read this Concept, and at its conclusion, you'll be ready to factor the above equation and solve it.

Watch This

James Sousa Example: Solve a Trig Equation by Factoring

Guidance

You have no doubt had experience with factoring. You have probably factored equations when looking for the possible values of some variable, such as "x". It might interest you to find out that you can use the same factoring method for more than just a variable that is a number. You can factor trigonometric equations to find the possible values the function can take to satisfy an equation.

Algebraic skills like factoring and substitution that are used to solve various equations are very useful when solving trigonometric equations. As with algebraic expressions, one must be careful to avoid dividing by zero during these maneuvers.

Example A

Solve 2sin2x3sinx+1=0 for 0<x2π.

Solution:

x2sin2x3sinx+1=0Factor this like a quadratic equation(2sinx1)(sinx1)=0     2sinx1=0or  sinx1=0   2sinx=1 sinx=1  sinx=12  x=π2=π6 and x=5π6

x2sin2x3sinx+1=0Factor this like a quadratic equation(2sinx1)(sinx1)=0     2sinx1=0or  sinx1=0   2sinx=1 sinx=1  sinx=12  x=π2=π6 and x=5π6

Example B

Solve 2tanxsinx+2sinx=tanx+1 for all values of x.

Solution:

Pull out sinx

There is a common factor of (tanx+1)

Think of the (tanx+1) as (1)(tanx+1), which is why there is a 1 behind the 2sinx.

Example C

Solve 2sin2x+3sinx2=0 for all x,[0,π].

Solution:

x2sin2x+3sinx2=0Factor like a quadratic(2sinx1)(sinx+2)=0  2sinx1=0sinx+2=0 sinx=12  sinx=2=π6 and x=5π6 There is no solution because the range of sinx is [1,1].

x2sin2x+3sinx2=0Factor like a quadratic(2sinx1)(sinx+2)=0  2sinx1=0sinx+2=0 sinx=12  sinx=2=π6 and x=5π6 There is no solution because the range of sinx is [1,1].

Some trigonometric equations have no solutions. This means that there is no replacement for the variable that will result in a true expression.

Guided Practice

1. Solve the trigonometric equation 4sinxcosx+2cosx2sinx1=0 such that 0x<2π.

2. Solve tan2x=3tanx for x over [0,π].

3. Find all the solutions for the trigonometric equation 2sin2x43cosx4=0 over the interval [0,2π).

Solutions:

1. Use factoring by grouping.

2sinx+1=0or2cosx1=02sinx=12cosx=1  sinx=12cosx=12 x=7π6,11π6x=π3,5π3

2sinx+1=0or2cosx1=02sinx=12cosx=1  sinx=12cosx=12 x=7π6,11π6x=π3,5π3

2.

tan2xtan2x3tanxtanx(tanx3)tanxx=3tanx=0=0=0ortanx=3=0,π  x=1.25

tan2xtan2x3tanxtanx(tanx3)tanxx=3tanx=0=0=0ortanx=3=0,π  x=1.25

3.

2sin2x43cosx4=0

2(1cos2x4)3cosx4=0 22cos2x43cosx4=0 2cos2x4+3cosx42=0(2cosx41)(cosx4+2)=02cosx41=0orcosx4+2=0  2cosx4=1  cosx4=2cosx4=12x4=π3or5π3x=4π3  or20π3

2(1cos2x4)3cosx4=0 22cos2x43cosx4=0 2cos2x4+3cosx42=0(2cosx41)(cosx4+2)=02cosx41=0orcosx4+2=0  2cosx4=1  cosx4=2cosx4=12x4=π3or5π3x=4π3  or20π3

20π3 is eliminated as a solution because it is outside of the range and cosx4=2 will not generate any solutions because 2 is outside of the range of cosine. Therefore, the only solution is 4π3.

Concept Problem Solution

The equation you were given is

2sinxcosx=cosx

To solve this:

2sinxcosx=cosx

Subtract cosx from both sides and factor it out of the equation:

2sinxcosxcosx=0cosx(2sinx1)=0

Now set each factor equal to zero and solve. The first is cosx:

cosx=0x=π2,3π2

And now for the other term:

2sinx1=0sinx=12x=π6,5π6

Explore More

Solve each equation for x over the interval [0,2π).

  1. cos2(x)+2cos(x)+1=0
  2. 12sin(x)+sin2(x)=0
  3. 2cos(x)sin(x)cos(x)=0
  4. sin(x)tan2(x)sin(x)=0
  5. sec2(x)=4
  6. sin2(x)2sin(x)=0
  7. 3sin(x)=2cos2(x)
  8. 2sin2(x)+3sin(x)=2
  9. tan(x)sin2(x)=tan(x)
  10. 2sin2(x)+sin(x)=1
  11. 2cos(x)tan(x)tan(x)=0
  12. sin2(x)+sin(x)=2
  13. tan(x)(2cos2(x)+3cos(x)2)=0
  14. sin2(x)+1=2sin(x)
  15. 2cos2(x)3cos(x)=2

Vocabulary

Factoring

Factoring

Factoring is the process of dividing a number or expression into a product of smaller numbers or expressions.

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Difficulty Level:

At Grade

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Date Created:

Sep 26, 2012

Last Modified:

Feb 26, 2015
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