1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

How would I solve sin2Ѳ+sin3Ѳ=0?

  1. Sep 1, 2014 #1
    EDIT: Please delete...was very confused and have now found solution!
     
    Last edited: Sep 1, 2014
  2. jcsd
  3. Sep 2, 2014 #2

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    How about posting the answer instead, then someone else confused can benefit from your confusion ;)
     
  4. Sep 2, 2014 #3
    Sure!
    Use sin(A+B)/2*cos(A-B)/2
    =2sin (3x+2x)/2*cos (2x-3x)/2
    2sin(5x)=0
    Thus sinx=0
    x=0, 180+2 (pi)n
    2cos (-x)/2
    Thus cosx=1/2
    x=90+2 (pi)n
     
  5. Sep 2, 2014 #4

    ehild

    User Avatar
    Homework Helper
    Gold Member

    Do you mean that Θ=pi/2 is a solution?
    But sin(2Θ)+sin(3Θ)=sin(180°)+sin(270°)=-1.

    And what about Θ=72°? Is not it a solution, too?

    ehild
     
  6. Sep 2, 2014 #5
    Well, the solutions manual is getting:
    Θ=0, 2∏/5, 4∏/5, ∏, 6∏/5, 8∏/5
    The interval is also 0 ≤ θ ≤2∏

    I forgot to include the interval before, but could figure it out without the interval. Now I don't know where to go with the problem as it looks like I'm solving it completely wrong.
     
    Last edited: Sep 2, 2014
  7. Sep 2, 2014 #6

    ehild

    User Avatar
    Homework Helper
    Gold Member

    No, you started very well, it was a splendid idea.
    But you miss some parentheses. It should be 2sin [(3x+2x)/2]*cos [(2x-3x)/2]
    So sin(5x/2)=0, and from this does not follow that sinx=0. Instead, sin(5x/2)=0 that is, 5x/2=kπ. x=???

    Do the same with the other factor, cos(x/2).

    ehild
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: How would I solve sin2Ѳ+sin3Ѳ=0?
  1. How can I solve it? (Replies: 6)

  2. How do I solve this? (Replies: 1)

  3. How i solve this (Replies: 7)

Loading...