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: 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
    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
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook