Solving for X in a Trig Equation: 5=2sinX + cosX

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In summary: Acos(x-\theta)= Acos(x)Acos(\theta)+ Asin(x)Asin(\theta) = A^2cos(x-\theta)+ 0sin(x-\theta) which gives A= \sqrt{13} and cos(\theta)= 3/ \sqrt{13} = 3\sqrt{13}/13 so you are looking for cos(x-\theta)= 3 \sqrt{13}. So what is x-\theta?In summary, the conversation is discussing how to solve for X, an angle, in an equation that involves sin and cos. Various methods are suggested, including using trigonometric identities, rewriting the equation, and substituting for sin and cos.
  • #1
21385
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How can you solve for X, an angle, if you get an equation like this?

5=2sinX + cosX

I couldn't think of any trig identities that can solve this, even though this may be extremely easy.

Can someone show me? Thanks
 
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  • #2
Well, what you cay say immediately is that the solution is going to be complex. What are the maximum values of sin and cos on the reals? :wink:
 
  • #3
srry about that...i just randomly put a few numbers down...

If the equation is like:: 2=3sinX+cosX

How would you solve that?
 
  • #5
21385 said:
srry about that...i just randomly put a few numbers down...

If the equation is like:: 2=3sinX+cosX

How would you solve that?
That's a bit complicated but you could do this: Let y= cos(X). Since [itex]sin(x)= \sqrt{1- cos^2(x)}[/itex] we have [itex]2= 3\sqrt{1- y^2}+ y[/itex]. Rewrite that as [itex]3\sqrt{1- y^2}= 2- y[/itex] and square both sides: [itex]9(1- y^2)= 4- 4y+ y^2[/itex] or [itex]9- 9y^2= 4- 4y+ y^2[/itex] so [itex]10y^2- 4y- 5= 0[/itex]. Solve that using the quadratic formula to find y= cos(X) and take the arcsine to find X.
 
  • #6
Substitute [tex]sin(x)= \sqrt{1- cos^2(x)}[/tex] and solve the quadratic for [tex]cos(x)[/tex]. Then take the arcsine of you two solutions. Basically what HallsofIvy said.
 
  • #7
or try to substitute :

sin(x) = 2tan(x/2)/1+tan^2(x/2)
and
cos(x)= (1-tan^2(x/2))/1+tan^2(x/2)

it might solve the problem !
 
  • #8
Why not get sin on one side and cos on the other side. Square both sides (possibly introducing extraneous roots) and substituting for either sin^2 or cos^2 (using sin^2+cos^2=1). This would result in a quadratic equation. Check the solutions.
 
  • #9
You could write it in the form of [tex]2=Acos(x-\theta)[/tex]
 

FAQ: Solving for X in a Trig Equation: 5=2sinX + cosX

1. What is the process for solving for X in a trig equation?

The process for solving for X in a trig equation involves using trigonometric identities and algebraic manipulations to isolate the variable, X, on one side of the equation. This typically requires using inverse trigonometric functions, such as arcsin, arccos, and arctan.

2. How do I know which trig identities to use when solving for X?

To determine which trig identities to use, you must first identify the type of trig equation you are dealing with. This can be done by looking at the given equation and seeing if it contains any trigonometric functions, such as sin, cos, or tan. Once you know the type of equation, you can then use the appropriate identities to manipulate the equation and solve for X.

3. Are there any special cases I should be aware of when solving for X in a trig equation?

Yes, there are a few special cases to be aware of when solving for X in a trig equation. These include equations with multiple angles, equations with fractions, and equations with multiple solutions. It is important to carefully consider these cases and use the appropriate techniques to solve for X.

4. Can I use a calculator to solve for X in a trig equation?

Yes, a calculator can be a helpful tool when solving for X in a trig equation. However, it is important to remember that calculators may not always give exact solutions and may round off decimal values. It is also important to check your answer by plugging it back into the original equation to ensure it is correct.

5. Are there any tips for solving for X in a trig equation more efficiently?

One tip for solving for X in a trig equation more efficiently is to try to simplify the equation as much as possible before using any trig identities. This can help reduce the number of steps needed to solve for X. Additionally, it can be helpful to have a good understanding of basic trigonometric identities and how they can be used to manipulate equations.

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