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: Maths Higher Tomorrow!

  1. May 20, 2004 #1
    I have the biggest exam of my life tomorrow -_-

    and i need to know how to do these questions :

    The point Q divides the line joining P(-1, -1, 0) to R(5, 2, -3) in the ratio 2:1. Find the co-ordinates of Q


    The answer is (3, 1, -2)

    Help is appreciated.
  2. jcsd
  3. May 20, 2004 #2


    User Avatar
    Science Advisor
    Homework Helper

    [tex]\mbox{Let }\vec{Q}\ =\ (a,\ b,\ c)[/tex]
    [tex]\vec{Q}\ -\ \vec{P}\ =\ 2(\vec{R}\ -\ \vec{Q})[/tex]
    [tex](a,\ b,\ c)\ -\ (-1,\ -1,\ 0)\ =\ 2[(5,\ 2,\ -3)\ -\ (a,\ b,\ c)][/tex]
    [tex]a\ +\ 1\ =\ 10\ -\ 2a[/tex]

    From here, you should be able to figure out two more equations, and should find them very easy to solve, giving you a, b, and c, the co-ordinates of Q.
  4. May 24, 2004 #3
    Disagree with the symbol of AKG, remember POIN is a non-direction Quantity, so that u cant use the VECTOR symbol . And your answer is right. When u take Q-P u will get the vector(PQ) , and the same with R-Q :)
  5. May 24, 2004 #4


    User Avatar
    Science Advisor
    Homework Helper

    Every point in [itex]\mathbb R^3[/itex] has a vector representation. The point [itex]Q(a, b, c)[/itex] has a representation [itex]\vec Q = (a, b, c) = a\mathbf i + b\mathbf j + c\mathbf k[/itex]. It made more sense to work with the vectors than to work with the points.
  6. May 24, 2004 #5
    I think
    1/If we get Q(a,b,c) >> a,b,c are the co-ordinate of the POIN Q
    2/If we get vector(Q(a,b,c)) >> a,b,c are the co-ordinate of vector(OQ), with O(0,0,0)

    I agree with you 100% in this kind, may be i understand Q = POINT, but u want to mean Q = vector in your solving :)
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook