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!

Homework Help: Trigonometric Equations

  1. Jan 8, 2008 #1
    1. The problem statement, all variables and given/known data
    sinx + cosx = 0

    2. Relevant equations

    3. The attempt at a solution

    sinx + cosx = 0

    sinx = -cosx
    sinx = (+/-) sqrt(sin^2x - 1)
    (sinx)^2 = (+/-) sqrt(sin^2x - 1)^2
    sinx = sin^2x - 1

    Not really too sure what to do from here on.

    The answer is 3[tex]\pi[/tex]/4 and 7[tex]\pi[/tex]/4.

    I appreciate the help, thanks in advance.
  2. jcsd
  3. Jan 8, 2008 #2
    [tex]sin x = -\cos x[/tex]

    Divide by cosx, what identity do you get?
  4. Jan 8, 2008 #3


    User Avatar
    Science Advisor

    Or, since you have gone to all the work of getting to [itex]sin(x)= sin^2(x)- 1[/tex], let y= sin(x) and rewrite it as y^2- y- 1= 0. solve that quadratic equation for y and then solve sin(x)= y.
    Last edited by a moderator: Jan 9, 2008
  5. Jan 8, 2008 #4
    Thank you for the assistance, I can't believe I missed that!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook