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Homework Help: Trig Identities

  1. Apr 27, 2016 #1
    1. The problem statement, all variables and given/known data
    1/1+cosx = csc^2x - cscxcotx

    2. Relevant equations

    3. The attempt at a solution
    R.S= csc^2x - cscxcotx
    = (1/sin^2x) - (1/sinx * cosx/sinx)
    = (1/sin^2x) - (cosx/sinx^2x)
    = 1-cosx / sin^2x
    = 1 - cosx / 1 - cos^2x
    = 1 / 1 - cos^2x

    It does not match the left side and I am unsure of what I did incorrectly.
  2. jcsd
  3. Apr 27, 2016 #2
    perhaps a simple typo error

    see for yourself /check
  4. Apr 27, 2016 #3
    Where in the left side? Or in my actual work?
  5. Apr 27, 2016 #4
    you have on the left side
    and on right side you got ( 1-cosx) / (1-cos^2x);
    the denominator can be written as (1+cosx).(1- cosx) using identity a^2- b^2 = (a+b).(a-b)
    so the numerator gets cancelled and you get 1/(1+cosx)
    am i right?
  6. Apr 27, 2016 #5


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    This step here is the problem:

    How did you go from

    ##=\frac{1-cos(x)}{1-cos^2(x)}## to

    ##=\frac{1}{1-cos^2(x)}## ?
  7. Apr 27, 2016 #6
    I got rid of one of the cos from numerator then got rid of one from denominator.
  8. Apr 27, 2016 #7


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    You certainly did not do that since the denominator did not change. But even if you had it would have been invalid.
    In a fraction, you cannot add something to the numerator and balance that by adding the same thing to the denominator. All you can do is multiply (or divide) the numerator as a whole by something and do exactly the same to the denominator. This is what drvrm did in post #4. Even then, you do need to be a bit careful. If the thing you multiply or divide by might be zero under some circumstances then you should exclude that by writing something like "when ..... is not zero then...."
  9. Apr 27, 2016 #8
    Okay thank you. When doing trig identities you are able to work on both sides correct?
  10. Apr 27, 2016 #9


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    Not sure what you mean by that. If you mean starting with the thing to prove in the form f(x)=g(x) then applying the same operation to each side of that equation, and repeating the process until you get something that is clearly true, no. That is not a valid way to prove the original statement. The implication is in the wrong direction. It is saying "if the thing to be proved is true then ..." E.g. if you simply multiply both sides by zero you will get a true statement, but you have proved nothing.
    However, that process can be used to help you find a proof. Having reached a statement that is clearly true, you can then see if all the steps you took are reversible, and hence obtain the thing to be proved.
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