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: Dot Product Question, Perpendicular

  1. Sep 19, 2010 #1

    pretty much this is all i was given.
    I have no idea how to even approach it. I do not need an answer (would be nice though), just an idea on how to go about starting it.
  2. jcsd
  3. Sep 19, 2010 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Education Advisor

    If a vector x is perpendicular to (-3, 8, -1), you know x•(-3, 8, -1)=0. Write out the left-hand side, and solve the equation.
  4. Sep 20, 2010 #3


    User Avatar
    Science Advisor

    Since this is in R3 writing the vector x as <x, y, z>, that dot product will given you a single equation in the three unknowns, x, y, and z. You can solve for one of them in terms of other two. For example, you were to find that z= ax+ by, then you could write your vector as <x, y, z>= <x, y, ax+ by>= x<1, 0, a>+ y<0, 1, b>.

    (On the other hand, if you solve for x, say, as x= py+ qz, then you could write the vector as <x, y, z>= <py+ qz, y, z>= y<p, 1, 0>+ z<q, 0, 1>. There are an infinite number of correct solutions to this problem.)
  5. Sep 20, 2010 #4
    this makes sense thank you.
  6. Sep 20, 2010 #5
    anyone else have an input just so i can be sure
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook