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Trig identities.

  1. Dec 13, 2008 #1
    Hi, I'm confused about using trig identities.

    1. The problem statement, all variables and given/known data

    Match the trigonometric function with one of the following: (a) -1, (b) cos(x), (c) cotx (d) 1, (e) -tan(x), (f) sin(x)

    (1-cos^2x)(cscx)

    2. Relevant equations

    None that I know of.

    3. The attempt at a solution

    I multiply it through, which gives me:

    csc(x) - cos^2(x)(cscx)

    I divide out csc(x) which gives me:

    csc(x)(1 - cos^2(x)(1))

    (got me nowhere really)

    edit: I just realized that I can do

    1-cos^2(x)=sin^2(x)

    edit2: am I wrong in thinking that since cosecant is the reciprocal of sine, that in csc(sin^2x) one sine cancels out, leaving me with sin(x)?

    I might of just solved my own problem :P can anybody confirm? Thanks
     
    Last edited: Dec 13, 2008
  2. jcsd
  3. Dec 13, 2008 #2
    Yes, that looks right. Good job.
     
  4. Dec 13, 2008 #3

    HallsofIvy

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    Staff Emeritus
    Science Advisor

    My first thought with something like that would be to write everything in terms of sine and cosine. Here csc(x)= 1/sin(x) so that problem is (1- cos2(x)/sin(x)= 1/sin(x)- cos2(x)/sin(x). But cos2(x)= 1- sin2(x) so that second fraction is (1- sin2(x))/sin(x)= 1/sin(x)- sin(x).
    1/sin(x)- cos2(x)/sin(x)= 1/sin(x)- 1/sin(x)+ sin(x)= sin(x), just as you say.
     
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