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Homework Help: I stuck at this vector question

  1. Jul 20, 2010 #1
    If A = 5ax + 3ay + 2az
    B = -ax + 4ay + 6az
    C = 8ax + 2ay

    Find the value of m and n such that mA + nB + C is parallel to y-axis.

    My problem is I did not know how to determine the m and n value because I did not know what method can use to solve for mA + nB + C parallel to y axis. Does it means that the vector line of mA + nB + C must be a straight line parallel to y axis?

    mA + nB + C
    = (5m - n + 8)ax + (3m + 4n + 2)ay + (2m + 6n)az

    To use dot product means that (mA + nB + C) dot (??) = 1
    cross product means that (mA + nB + C) cross (??) = 0

    I dunno the ?? is stand for what.... can someone pls help me?
  2. jcsd
  3. Jul 21, 2010 #2
    mA + nB + C should be a vector that should have same direction as <0,1,0>.

    What that means is that other than y component not equals zero, x and z components should be zero.
  4. Jul 21, 2010 #3
    Thanks, now I am able to solve the question :D
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