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3.15: Triple-Angle Formulas and Linear Combinations

Difficulty Level: At Grade Created by: CK-12
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In other Concepts you've dealt with double angle formulas. This was useful for finding the value of an angle that was double your well known value. Now consider the idea of a "triple angle formula". If someone gave you a problem like this:

sin135

Could you compute its value?

Keep reading, and at the end of this Concept you'll know how to simplify equations such as this using the triple angle formula.

Watch This

Deriving a Triple Angle Formula

Guidance

Double angle formulas are great for computing the value of a trig function in certain cases. However, sometimes different multiples than two times and angle are desired. For example, it might be desirable to have three times the value of an angle to use as the argument of a trig function.

By combining the sum formula and the double angle formula, formulas for triple angles and more can be found.

Here, we take an equation which takes a linear combination of sine and cosine and converts it into a simpler cosine function.

Acosx+Bsinx=Ccos(xD), where C=A2+B2, cosD=AC and sinD=BC.

You can also use the TI-83 to solve trigonometric equations. It is sometimes easier than solving the equation algebraically. Just be careful with the directions and make sure your final answer is in the form that is called for. You calculator cannot put radians in terms of π.

Example A

Find the formula for sin3x

Solution: Use both the double angle formula and the sum formula.

sin3x=sin(2x+x)=sin(2x)cosx+cos(2x)sinx=(2sinxcosx)cosx+(cos2xsin2x)sinx=2sinxcos2x+cos2xsinxsin3x=3sinxcos2xsin3x=3sinx(1sin2x)sin3x=3sinx4sin3x

Example B

Transform 3cos2x4sin2x into the form Ccos(2xD)

Solution: A=3 and B=4, so C=32+(4)2=5. Therefore cosD=35 and sinD=45 which makes the reference angle is 53.1 or 0.927 radians. since cosine is positive and sine is negative, the angle must be a fourth quadrant angle. D must therefore be 306.9 or 5.36 radians. The final answer is 3cos2x4sin2x=5cos(2x5.36).

Example C

Solve sinx=2cosx such that 0x2π using a graphing calculator.

Solution: In y=, graph y1=sinx and y2=2cosx.

Next, use CALC to find the intersection points of the graphs.

Guided Practice

1. Transform  5cosx5sinx to the form Ccos(xD)

2. Transform 15cos3x8sin3x to the form Ccos(xD)

3. Derive a formula for tan4x.

Solutions:

1. If 5cosx5sinx, then A=5 and B=5. By the Pythagorean Theorem, C=52 and cosD=552=12=22. So, because B is negative, D is in Quadrant IV. Therefore, D=7π4. Our final answer is 52cos(x7π4).

2. If 15cos3x8sin3x, then A=15 and B=8. By the Pythagorean Theorem, C=17. Because A and B are both negative, D is in Quadrant III, which means D=cos1(1517)=0.49+π=3.63 rad. Our final answer is 17cos3(x3.63).

3.

tan4x=tan(2x+2x)=tan2x+tan2x1tan2xtan2x=2tan2x1tan22x=22tanx1tan2x1(2tanx1tan2x)2=4tanx1tan2x÷(1tan2x)24tan2x(1tan2x)2=4tanx1tan2x÷12tan2x+tan4x4tan2x(1tan2x)2=4tanx1tan2x(1tan2x)216tan2x+tan4x=4tanx4tan3x16tan2x+tan4x

Concept Problem Solution

Using the triple angle formula we learned in this Concept for the sine function, we can break the angle down into three times a well known angle:

sin3x=3sinx4sin3x

we can solve this problem.

sin(3×45)=3sin454sin345=3224(22)3=322(4(2)2/38)=32222=22

Explore More

Transform each expression to the form Ccos(xD).

  1. 3cosx2sinx
  2. 2cosxsinx
  3. 4cosx+5sinx
  4. 7cosx6sinx
  5. 11cosx+9sinx
  6. 14cosx+2sinx
  7. 2cosx4sinx

Derive a formula for each expression.

  1. sin4x
  2. cos6x
  3. cos4x
  4. csc2x
  5. cot2x

Find all solutions to each equation in the interval [0,2π).

  1. cosx+cos3x=0
  2. sin2x=cos3x
  3. cos2x+cos4x=0

Answers for Explore More Problems

To view the Explore More answers, open this PDF file and look for section 3.15. 

Vocabulary

Linear Combination

Linear Combination

A linear combination is a set of terms that are added or subtracted from each other with a multiplicative constant in front of each term.
Triple Angle Identity

Triple Angle Identity

A triple angle identity (also referred to as a triple angle formula) relates a trigonometric function of three times an argument to a set of trigonometric functions, each containing the original argument. Examples include: the Triple Angle Formula for Sine \text{sin} (3\theta) = 3 \text{sin} \theta - 4 \text{sin}^3 \theta, the Triple Angle Formula for Cosine \text{cos} (3 \theta) = -3 \text{ cos} \theta + 4 \text{ cos}^3 \theta, and the Triple Angle Formula for Tangent \text{tan} (3 \theta) = \frac{3 \text{ tan} \theta - \text{ tan}^3 \theta}{1 - 3 \text{ tan}^2 \theta}.

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Difficulty Level:

At Grade

Subjects:

Grades:

Date Created:

Sep 26, 2012

Last Modified:

Feb 26, 2015
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