Finding the smallest positive solution to trig equation

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Discussion Overview

The discussion revolves around finding the smallest positive solution to the trigonometric equation ##\sin 3x = \cos 7x##. Participants explore various methods to determine this solution, including theoretical reasoning and graphical analysis.

Discussion Character

  • Exploratory
  • Mathematical reasoning
  • Technical explanation

Main Points Raised

  • One participant suggests that since sine and cosine are equal when their arguments are complementary, setting up the equation ##3x + 7x = 90## leads to a solution of ##x = 9##.
  • Another participant points out that while ##x = 9## is a solution, the general solution for sine and cosine equality includes periodic terms, indicating that ##x = 45 + 180n## for integer n must be considered to find the smallest positive solution.
  • A different participant asserts that since both angles are in the first quadrant, the solution of ##x = 9## is likely the smallest, although they acknowledge that proving this may require additional work.
  • Further, this participant discusses the possibility of finding additional solutions by manipulating the equation using trigonometric identities and periodicity.
  • Another participant suggests that graphing the functions ##y = \sin(3x)## and ##y = \cos(7x)## could provide a visual confirmation of the smallest solution being ##x = 9##.

Areas of Agreement / Disagreement

Participants express differing views on the completeness of the solution process and the identification of the smallest positive solution. While some support the idea that ##x = 9## is the smallest, others emphasize the need to consider periodic solutions and additional angles.

Contextual Notes

There are unresolved aspects regarding the periodic nature of trigonometric functions and how they affect the identification of the smallest solution. The discussion also reflects varying levels of certainty about the completeness of the proposed solutions.

Mr Davis 97
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I have the equation ##\sin 3x = \cos 7x##, and, in degrees, I have to find the smallest positive solution.

Immediately, we can see that sin and cos are equal if their arguments are complements, so ##3x + 7x = 90##, which means that ##x = 9##.

I know that that is a correct solution, but how do I show that it is, in fact, the smallest positive solution?
 
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I would do it this way:
sin(x)=cos(x) if x=45 degrees. Of course, you knew that and could then get the correct solution. However, you are missing one part -- x=45+180n degrees where n is an integer. If you are looking for the smallest possible x, n would have to be zero.
 
An interesting problem. Since the angles are both in the first quadrant, I think you clearly have the smallest x. To prove it might take a little work, but it would take more effort to find the second smallest or 3rd smallest. (Solving the equivalent ## \sin(3x)=\sin(90-7x) ##, trigonometric identities allow 360 degrees to be input into either side and you have essentially the same equation ) e.g. If ## 3x=360+(90-7x) ## so that ## 10x=450 ## then x=## 45 ##. I found another solution, but is it the second smallest? editing... You can also add 720 or other multiples of 360 to either side, or you can do ## 180-\theta ## to either term inside the sine function without changing the equation...
 
Last edited:
A follow-on to post #3: Overlaying a graph of y =sin(3x) and a graph of y=cos(7x) is perhaps the quickest way to see the (approximate) solutions of this problem. It will show you that your x=9 is the smallest x.
 

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