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!

Trigonometric Identities

  1. Nov 29, 2011 #1
    Hi,

    I am given that, for π/2 < x < π, sin x = 2/√13

    a) Find Cos x
    b) Find tan 2 x

    So, what I did was: I drew a triangle and found that the missing side was equal to 3. From then, I deduced that cos x was equal to 3/√13

    The problem was however that the angle must lie between the values given above. What I did was I simply added a negative sign. Is that right?

    For part b I did sin 2 x / cos 2 x = tan 2 x and solved. Is that right? I got a negative answer too, which makes sense in terms of the unit circle.

    Thanks,
    Peter
     
  2. jcsd
  3. Nov 29, 2011 #2

    Mark44

    Staff: Mentor

    Yes.
    I don't think so. You know sin(x) and you have found cos(x), but you don't know sin(2x) or cos(2x).

    Use the double angle identity for tangent: tan(2x) = 2tanx/(1 - tan2x).
     
  4. Nov 30, 2011 #3
    Ah ok. I did the sin 2(x)/cos2(x) because I hadn't learned the tan identity and therefore didn't have it in my formula booklet. Maybe I had to know it and I didn't :redface:

    Thanks!
     
  5. Nov 30, 2011 #4

    Mark44

    Staff: Mentor

    Actually, what you started to do would have worked. Since you know both sin(x) and cos(x) you could have used them to get sin(2x) and cos(2x), and then evaluated sin(2x)/cos(2x). What I suggested is just more direct.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Trigonometric Identities
  1. Trigonometric Identity (Replies: 8)

  2. Trigonometric Identity (Replies: 8)

  3. Trigonometric identity (Replies: 1)

  4. Trigonometric identity (Replies: 7)

  5. Trigonometric Identity (Replies: 1)

Loading...