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

Trigonometric equations - strange results

  1. Apr 24, 2004 #1
    Greetings all,

    I am getting strange results when solving this trig equation. I seem to be able to calculate 4 out 7 of the correct angles but how do i calculate the others? Maybe my method is wrong...

    A = theta

    2sin2A = tanA,

    considering identities sin2A = 2sinAcosA and tanA = sinA/cosA

    2(2sinAcosA) = sinA/cosA

    4sinAcos^2A = sinA

    dividing by sinA both sides

    4cos^2A = 1
    cos^2A = 1/4
    cosA = 1/2
    cosA = -1/2

    A = 60
    A = 300
    A = 120
    A = 240

    missing angles 0, 180 and 360 ?? :confused:

    Any help would be much appreciated, thanks
  2. jcsd
  3. Apr 24, 2004 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    Dearly Missed

    When dividing with sin(A), you have assumed sin(A) not equal to zero.
  4. Apr 25, 2004 #3
    I think that you have to illustrate that

    [tex]4sin(A)cos^2(A) = sin(A)[/tex]

    [tex]4sin(A)cos^2(A) - sin(A) = 0[/tex]


    [tex] sin(A)(4cos^2(A) - 1) = 0[/tex]
  5. Apr 25, 2004 #4


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Or, alternatively, you have to break the problem up into two cases; case 1 is where sin A is 0, and case 2 is where sin A is not zero (and thus you can divide by sin A)

    But whatever you do, the point we're making is that, in general, you cannot divide by something that may be zero in your problem.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook