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Need Trigonometry help

  1. Aug 19, 2010 #1
    1. The problem statement, all variables and given/known data
    I can't find way to solve [tex]\frac{Sin25 * Sin65}{Cos 40}[/tex]



    2. Relevant equations



    3. The attempt at a solution
    I looked trough allformulas i have I came up with:
    [tex]\frac{Sin25 * Sin65}{Cos 40} = \frac{Sin25 * Cos25}{Cos 40}[/tex]

    But then found that formula is:
    [tex]Sin 2a =2{Sina * Cosa}[/tex]

    Which means what i tried above doesn't work.
     
  2. jcsd
  3. Aug 19, 2010 #2

    rock.freak667

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    Try writing sin65 as sin(25+40).
     
  4. Aug 19, 2010 #3

    Mentallic

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    I don't see how that would help.

    Why not? You gave up on this idea too quickly. Just one extra step and everything will be blatantly obvious.
     
  5. Aug 19, 2010 #4

    rock.freak667

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    I wasn't too sure if it would, I was hoping the cos40 would cancel on expansion.
     
  6. Aug 19, 2010 #5

    Mentallic

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    Yeah I know where you were coming from :smile: I actually thought it would work too so I quickly scribbled some stuff down but it didn't seem to go anywhere far.
     
  7. Aug 19, 2010 #6

    ehild

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    Some more hint: 40=65-25; 90=65+25

    ehild
     
  8. Aug 19, 2010 #7
    thanx.

    Is there any goog tuts for learning trigonometry on my own? My textbook is terrible, it starts with hard things soon as explanations part is over. I need something that begins with easier things (but not too easy). I also need good reference for line (Ax+By+C=0) and circle ((x - a) ^2 + (y - b)^2 = r^2).
     
  9. Aug 19, 2010 #8

    vela

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    I haven't actually looked through it, but the Schaum's outline for trig is probably a good resource to complement your textbook. Schaum's outlines typically summarize the material and have worked example as well as practice problems with answers.
     
  10. Aug 20, 2010 #9

    Mentallic

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    Sorry what do you mean by this?
     
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