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Thread: Proving an identity

  1. #1
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    Question Proving an identity

    Hi,

    I hope someone can help. I'm trying to understand the process of proving an identity.

    Let's say that you were trying to prove the following equation: sin2x/(1+cos2x) = tanx

    I know that the equation is an identity since when I simplify the left-hand side, it is equivalent to the right-hand side.

    But let's say that I replaced x with pi/4, and still showed that both sides were equivalent - would using only one value be enough to prove that the equation is an identity? I would think that by proving the equation using only one value (e.g. pi/4), would not be enough to prove that the equation is an identity. I thought that x takes into account all values, so it is more accurate than merely subbing in pi/4.

    The definition of a trigonometric identity is as follows:

    Proving an identity-screen-shot-2017-06-06-1.37.19-pm.png
    I want to place emphasis on the fact that the definition says that the solution set for trigonometric identities is all real numbers (and not including the solutions that are undefined). So with that being said, if x was only equal to pi/4, then how could the equation possibly be an identity?

    I would appreciate some guidance.

    Sincerely,
    Olivia
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  2. #2
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    Re: Proving an identity

    Quote Originally Posted by otownsend View Post
    Let's say that you were trying to prove the following equation: sin2x/(1+cos2x) = tanx

    I know that the equation is an identity since when I simplify the left-hand side, it is equivalent to the right-hand side.
    But let's say that I replaced x with pi/4, and still showed that both sides were equivalent - would using only one value be enough to prove that the equation is an identity? I would think that by proving the equation using only one value (e.g. pi/4), would not be enough to prove that the equation is an identity. I thought that x takes into account all values, so it is more accurate than merely subbing in pi/4.

    The definition of a trigonometric identity is as follows:

    Click image for larger version. 

Name:	Screen Shot 2017-06-06 at 1.37.19 PM.png 
Views:	7 
Size:	37.4 KB 
ID:	37734
    I want to place emphasis on the fact that the definition says that the solution set for trigonometric identities is all real numbers (and not including the solutions that are undefined). So with that being said, if x was only equal to pi/4, then how could the equation possibly be an identity?
    Note that $2\left( {\dfrac{\pi }{4}} \right) = \left( {\dfrac{\pi }{2}} \right)$ so

    $ \begin{align*}\dfrac{\sin \left( {\frac{\pi }{2}} \right)}{1-\cos \left( {\frac{\pi }{2}} \right)}&=\dfrac{1}{1-0}\\&=1\\&=\tan \left( {\frac{\pi }{4}} \right) \end{align*}$
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  3. #3
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    Re: Proving an identity

    They gave an example of it working. It may be left up to the reader to prove the identity. Such as this:

    $\dfrac{\sin(2x)}{1+\cos(2x)} = \dfrac{2\sin x\cos x}{1+2\cos^2 x-1} = \dfrac{\sin x \cancel{2\cos x}}{\cancel{2\cos x} \cos x} = \dfrac{\sin x}{\cos x} = \tan x$

    This proves the identity.
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    Re: Proving an identity

    Thank you for responding... however, I'm more wondering whether it is incorrect style to prove an identity by solving for a specific variable as opposed to just x?
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    Re: Proving an identity

    Quote Originally Posted by otownsend View Post
    Thank you for responding... however, I'm more wondering whether it is incorrect style to prove an identity by solving for a specific variable as opposed to just x?
    A statement, an identity, is never proved by example.
    An example can disprove a statement called a counter-example; but it can never ever prove a statement.
    Last edited by Plato; Jun 6th 2017 at 02:29 PM.
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    Re: Proving an identity

    What if the identity held true to an example such as when x = pi/4? What would that be communicating? Based on what you have told me, I would think that this would not be enough to claim the whole equation for all cases as an identity. Please let me know if my thinking about this is right...
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    Re: Proving an identity

    Quote Originally Posted by otownsend View Post
    What if the identity held true to an example such as when x = pi/4? What would that be communicating? Based on what you have told me, I would think that this would not be enough to claim the whole equation for all cases as an identity. Please let me know if my thinking about this is right...
    No, it just proves that the world is flat.
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    Re: Proving an identity

    Right, and it also proves that global warming isn't real.
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    Re: Proving an identity

    Quote Originally Posted by otownsend View Post
    Right, and it also proves that global warming isn't real.
    Well again, ask a dumb question get a dumb answer.
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  10. #10
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    Re: Proving an identity

    Well you have to give me some slack since I'm learning pre-calculus without any official teacher or student body. My learning is 100% self-guided.

    I occasionally ask what you define as a "stupid questions" since I don't have anyone to guide me, and so I therefore need reassurance in my thought-process, even if the questions may seem obvious sometimes. I just want to be confident in my thinking. As a matter of fact, the result of me making an effort to get down to the facts has lead to me getting close to perfect on all my tests. However, I do understand that asking questions that may seem obvious could be a sign that I'm not critically thinking about what I'm asking - that occasionally is the case, and I'm sorry for doing that. I'm genuinely trying my best here to learn and so I would appreciate support.

    I really appreciate the MHF community and the help which the pre-calc forum has provided me so far in my learning. I would hate to stop using it.

    Please don't take this as a negative message! Again, I REALLY appreciate your help so far (not sarcastic). I just want to shed some light on my situation so you can hopefully respond more reasonably when I ask what seems to you as an obvious question.
    Last edited by otownsend; Jun 6th 2017 at 06:40 PM.
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