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Math Help - Compute the values

  1. #1
    Member akhayoon's Avatar
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    Compute the values

    for ||3X-4Y||

    knowing that ||X||=2 AND ||Y||=1 and X.Y=-1


    and then compute for

    (3X+4Y).(X-5Y)


    apparently the answer for the first is square root of 76

    and the second is 1


    I tried doing this the way the book tells me but I don't get the right answer at ALL!
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  2. #2
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    \begin{array}{rcl}<br />
 \left\| {3X - 4Y} \right\|^2 & = & \left( {3X - 4Y} \right) \cdot \left( {3X - 4Y} \right) \\ <br />
  & = & 9\left\| X \right\|^2  + 16\left\| Y \right\|^2 - 24\left( {X \cdot Y} \right) \\  \end{array}

    Just make the substitutions and the squareroot of both sides.
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