Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Unit Circle

  1. Sep 5, 2003 #1
    How do I derive the formula 'Sin(a+b)=sinacosb + cosasinb' from the unit circle. Any ideas would be appreaciated our study group tried and failed.
     
  2. jcsd
  3. Sep 5, 2003 #2
    http://www.clowder.net/hop/cos(a+b).html
     
    Last edited: Sep 5, 2003
  4. Sep 5, 2003 #3

    HallsofIvy

    User Avatar
    Staff Emeritus
    Science Advisor

    That's hard! (Well, more tedious than hard.)

    It's a little easier to prove 'Sin(a-b)=sinacosb - cosasinb'
    (and then change the sign on b).

    The basic idea is to set up the points whose coordinates are
    (cos(a),sin(a)) (i.e. the point a distance a from (0,0) measured along the circle) and (cos(b),sin(b)) and calculate the straight line distance between them (the arc distance, along the circle, is a-b, of course.) Now mark the point whose arc length from (1,0) is also a-b: it's coordinates are (cos(a-b), sin(a-b)) and calculate the straight line distance beween it and (1,0). Since the arclengths are the same, the lengths of these chords are the same. Set the two calculations equal and "grind".
     
  5. Sep 5, 2003 #4

    HallsofIvy

    User Avatar
    Staff Emeritus
    Science Advisor

    Having actually sat down and done the calculation, I find that my suggestion gives the cos(x+y) and cos(x-y). I'm going to have to think about how to get sin(x+y)!!
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Unit Circle
  1. Trig Unit Circle Why? (Replies: 6)

Loading...