Solving Trigonomic Inequalites

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In summary, to find where cosθ = -0.8660, you can use the formula cosθ + 0.8660 = 0 and then solve for θ using a calculator or recognizing that 0.8660 is equivalent to \frac{1}{2} \sqrt(3). This will give you the "principal value" of θ. Then, use the fact that cosine is periodic with a period of 360 degrees and that cos(360-t) = cos(t) to find all other values of θ within the given range (0 degrees ≤ θ < 540 degrees).
  • #1
05holtel
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Find where cosθ = -0.8660, where (0 degrees ≤ θ < 540 degrees)

I am not sure how to solve this problem

I did: cosθ + 0.8660 = 0

Then I am not sure what to do
 
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  • #2
Well, use a calculator! That will give you the "principal value". Then use the fact that cosine is periodic with period 360 degrees and that cos(360- t)= cos(t) to find all other values.
 
  • #3
[tex]\cos \theta = -0.8660[/tex]
[tex]\theta = \arccos -0.8660[/tex]

That's as far as you can go if you want to stay exact. But you can calculate the answer using a calculator (there is probably something like 'cos-1' key on it, that's the arccos key.)

However, if you've worked with trigonometry for some time you will find that you recognize some numbers. For example: 0.8660 looks like it's probably meant to be [tex]\frac{1}{2} \sqrt(3) = 0.866025404...[/tex]

The arccos of -1/2 sqrt(3) however IS exact (try it on your calc!):

Note, this is only one answer. If you allow theta to be in the range you specified you get more answers!
 

1. What are trigonometric inequalities?

Trigonometric inequalities are inequalities that involve trigonometric functions, such as sine, cosine, and tangent. They often involve finding the range of values for which the inequality is true.

2. How do you solve trigonometric inequalities?

To solve trigonometric inequalities, you can use algebraic techniques, such as factoring and finding common denominators, as well as properties of trigonometric functions, such as the unit circle and the periodicity of functions. You can also use graphing techniques to visualize the solutions.

3. What are the common mistakes when solving trigonometric inequalities?

Some common mistakes when solving trigonometric inequalities include forgetting to check for extraneous solutions, using the wrong interval notation, and forgetting to consider the periodicity of trigonometric functions. It is important to double check your work and make sure it aligns with the given inequality.

4. How do you know if a solution to a trigonometric inequality is inclusive or exclusive?

Inclusive solutions, where the endpoint values are included in the solution set, are represented with a ≤ or ≥ sign. Exclusive solutions, where the endpoint values are not included, are represented with a < or > sign. It is important to pay attention to the given inequality and use the correct notation when expressing your final solution.

5. Can you use a calculator to solve trigonometric inequalities?

While a calculator can be helpful in graphing and visualizing solutions to trigonometric inequalities, it is important to remember that it is not a substitute for understanding the concepts and techniques used to solve them. It is always recommended to show your work and use the calculator as a tool, rather than relying on it solely for the solution.

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