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: Solving an Equation with Cosine

  1. Feb 18, 2009 #1
    1.x = rcos(a)cos(a)

    x is known
    r is known

    I cant seem to get this!

    x = -2
    r = 7

    therefore

    -2/7 = cos(a)*cos(a)

    so..

    -2/7 = cos^2(a)

    i know cos^2(a) = 1 + cos2a / 2

    -2/7 = 1 + cos2a / 2

    so
    -4/7 -1 = cos2a

    I cannot take cos inverse of this value since it is "greater" than 1 (-1.57)

    the book says the angle is 106.6 degrees!!!

    HELP!




    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Feb 18, 2009 #2

    Tom Mattson

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Here's your problem right here:



    Nope, it's [itex]\cos^2(a)=(1+\cos(2a))/2[/itex]. Both terms are divided by 2.
     
  4. Feb 18, 2009 #3
    yea sorry thats what i meant

    but it still doesnt work out..
     
  5. Feb 18, 2009 #4

    Tom Mattson

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Egad, you have an even more serious problem that I missed.

    You can't square cos(a) and get a negative number. This equation has no solution in the real numbers.
     
  6. Feb 18, 2009 #5
    Who is Egad?


    EDITED!!!

    NEVERMIND I figured it out, i was interpreting the solution incorrectly...

    Thanks anyways!!
     
  7. Feb 19, 2009 #6

    Tom Mattson

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    :rofl: "egad" is an exclamation like "crikey" or "blimey".
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook