1. Not finding help here? Sign up for a free 30min 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!

Help on another algebra problem

  1. Oct 23, 2005 #1
    The Cosine of the angle between a and b is 4/21. Find p if a = 6i + 3j -2k and b = -2i + pj - 4k.

    i started off with the general equation a . b = |a| |b| cosX where cosX is 4/21
    since i, j, k all mean 1. i would get two vectors
    a = (6,3,-2) and b = (-2, p, -4)

    then i would take the magnitude of both a and b.

    then i would take the dot product of a and b.

    then i would sub them into the equation.

    and then i ended up solving using the quadratic equation, but when i solved it, i ended up getting two answers and if i sub it back into the dot products and stuff it wouldn't equal.. LS = RS...

    i dont kno if this is how i shouold have done it ?
    thnx
     
  2. jcsd
  3. Oct 23, 2005 #2

    HallsofIvy

    User Avatar
    Staff Emeritus
    Science Advisor

    Yes, that is how you should do it.

    I don't know what you mean by "when i solved it, i ended up getting two answers and if i sub it back into the dot products and stuff it wouldn't equal.. LS = RS...". Of course, when you square an equation you may introduce a new solution that doesn't satisfy the original equation but one of your two solutions should.
     
  4. Oct 23, 2005 #3
    alright thanks
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?