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Homework Help: Compute angles, trignometry/pre-calc?

  1. Mar 6, 2013 #1
    1. The problem statement, all variables and given/known data
    I'm having a problem with this. I was told to use the difference of sums formula. I'm not exactly sure of what that is/means.

    2. Relevant equations
    Difference of sums?

    3. The attempt at a solution
    I'm thinking that Cos(a)=1/3 refers to 1 being the adjacent angle and 3 is the hypotenuse. Therefore the opposite angle equals to sqrt(32-12). Or does 1/3 somehow correspond to an angle? a or alpha should be 30 degrees then right? I need a lead-in. From there, I think that I'll be able to take it form there. Than you in advance.

    Attached Files:

  2. jcsd
  3. Mar 6, 2013 #2


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    Staff Emeritus
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    You are making this more complicated than necessary:

    cos(a) = 1/3 = 0.3333...

    Sometimes a cigar is just a cigar.
  4. Mar 6, 2013 #3
    Lol, this is why I often miss a lot of points on tests, my minds always on "it's not that easy mode". But how does that go into helping me figure out the other problems(In the picture)? Being on that mode again, I'm thinking that it's not as easy as just subtracting and replacing the signs. Thank you.
  5. Mar 6, 2013 #4


    Staff: Mentor

    No, the opposite side is sqrt(32-12). To get the opposite angle, find angle a using the inverse cosine function, and then subtract a from ##\pi##/2.
  6. Mar 6, 2013 #5


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    The beginning of the problem states:
    ... which makes no sense. The terminal side of an angle of π radians (or 180°) would be on the negative x-axis, between the 2nd and 3rd quadrants. Do you mean to say,
    "Given an angle a in the first quadrant..."?
  7. Mar 6, 2013 #6
    Yes, sorry about that. It was a stupid copy and paste of Pi, I mean a.
  8. Mar 7, 2013 #7


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    Staff Emeritus
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    You do know the sum and difference formulas for calculating the sine and cosine of angle sums and angle differences, don't you?
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