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Homework Help: Another trig. question

  1. Dec 11, 2009 #1
    1. The problem statement, all variables and given/known data
    Given that 5sec^2+3tan^2 = 9
    find sinx value

    2. Relevant equations
    trig. identities


    3. The attempt at a solution
    i started off by subbing. in sec2= 1+tan^2
    =>
    5(1+tan^2)+3tan^2= 9
    5+8tan^2x = 9
    8tan^2 = 4
    tan^2 = 1/2 <--- no idea of this bit, as i am supposed to find sin x value =/ , hmm i even wonder if this right.
     
  2. jcsd
  3. Dec 11, 2009 #2

    rock.freak667

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    Yes it is correct. Take the square root and find the angles for the two values.
     
  4. Dec 11, 2009 #3
    tanx= [tex]\sqrt{}1/2[/tex]
    x= inverse. sin [tex]\sqrt{}1/2[/tex] is that how i get values for sin?
     
  5. Dec 11, 2009 #4

    rock.freak667

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    You will have two values, x2=a ⇒ x=±√a

    Also if tanx=y, then x=tan-1(y). Where did you get inverse sine from?
     
  6. Dec 11, 2009 #5
    Exactly lol thats what i thought.. the question wants me to get sinx values if possible :9.
     
  7. Dec 11, 2009 #6

    rock.freak667

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    Sorry, I mis-read the question. While you can just find the values for x, and then find sinx.

    You can also use tanx=sinx/cosx and then use cos2x=1-sin2x and solve for sinx.
     
  8. Dec 12, 2009 #7
    ah k, thanks for helping me out :).
     
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