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Homework Help: Does anyone understand this answer? proving sin(A-B)

  1. Jan 17, 2008 #1
    I had no clue how to do this and could only grasp the first basic parts but even then i couldn't get it right and in the end i had to look at the answer, which confused me even more. So i was wondering if anyone else understands this, and if so could you describe it in words please?

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

    2. Relevant equations
    As above

    3. The attempt at a solution

    I understand the first part, getting cos B from the graph...yet when it gets to the other part '...angle in the bottom right hand corner of the triangle is PI - A...' i don't understand how they came to that...but i was wondering, the sin (PI/2 - A) seems to be related to a trigonometric identity? If someone could explain it that would be great.
  2. jcsd
  3. Jan 17, 2008 #2


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    Science Advisor

    The angle in the bottom right hand corner is NOT [itex]\pi- A[/itex]. The two non-right angles in a right triangle must add to [itex]\pi/2[/itex]. Since the whole angle in the upper left is A, the angle in the bottom righthand corner is [itex]\pi/2- A[/itex].
  4. Jan 17, 2008 #3
    Thank you for that, it clears some of the confusion, thanks.
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