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Sum and differences identities equations

  1. Mar 24, 2014 #1
    1. The problem statement, all variables and given/known data
    Which is equivalent to: cos(∏/2 + x) - cos(∏/2 - x)?
    A) -2cos(x)
    B) -2
    C) 0
    D)-2sin(x)


    2. Relevant equations
    Cos (A-B)


    3. The attempt at a solution
    I am totally stuck :( please help!
     
  2. jcsd
  3. Mar 24, 2014 #2

    Dick

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    You mentioned the sum and difference formulas. I think you should try to use them.
     
  4. Mar 24, 2014 #3
    I have tried to use them but i totally get stuck like i dont know which is a or b and we are only given cos (a-b) = cosAcosB + sinAsinB
     
  5. Mar 24, 2014 #4

    Dick

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    In cos(pi/2-x) the 'a' is pi/2 and the 'b' is x. So what does that turn into? What are cos(pi/2) and sin(pi/2)?
     
  6. Mar 24, 2014 #5
    Cos (pi/2) =0
    Sin(pi/2)= 1
     
  7. Mar 24, 2014 #6

    Dick

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    Ok, do go on. So what is cos(pi/2-x)?
     
  8. Mar 24, 2014 #7
    How come the b is not a -x ?
     
  9. Mar 24, 2014 #8
    So cos( 0-x)?
     
  10. Mar 24, 2014 #9

    SteamKing

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    It doesn't matter. Pick one angle and make it A. The other then must be B.
     
  11. Mar 24, 2014 #10

    Dick

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    Why would you think that?? If you have a formula for cos(a-b) and you want to apply it to cos(pi/2-x) then you should put a=pi/2 and b=x. Your formula already has the '-' in it. Just use the formula and stop trying to double think it.
     
  12. Mar 24, 2014 #11
    But the formula also has pi/2 + x theres 2 A's and 2 B's
     
  13. Mar 24, 2014 #12

    Dick

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    Use the formula SEPARATELY for each one. You can choose them differently for cos(pi/2-x) and cos(pi/2+x). Finish cos(pi/2-x) first, then worry about the other one.
     
  14. Mar 24, 2014 #13
    Okay i thought they had to be the same because the teacher didn't show us any other way thanks!
     
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