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Planes and lines Q

  1. Oct 26, 2013 #1
    write the equation of the plane x - 2y - 2z = 27 in the form [tex] r.\hat{n} = d [/tex] done

    write down the distance of the origin from the plane and show that the point which is the reflection of the origin is (6, -12, -12) done

    A second point P has coordinates (-3,2,1). Find the direction cosines of the reflection of the line OP in the plane.

    I need help with the last part. 1. How would I go about finding the line which is reflected? Also, how do I find the direction of cosines of a line - is it the just direction of cosines of the direction of the line?
     
  2. jcsd
  3. Oct 26, 2013 #2

    UltrafastPED

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  4. Oct 26, 2013 #3
    ok here is what i have so far

    I thought I would use the point P and reflect it in the plane, doing this I got (5,-14,-15) as P' (reflected P). Then I created a line through O and P and saw that it intersected the line at (9,-6,-3) (call it A) then I found the direction of P' and where OP and the plane intersected and got (-4,8,-12) and used this to get find the cosines

    is this a correct method? for instance, I done: P' - A to find the direction of the line, if I had done A-P' I would get a different value for the cosines no?
     
  5. Oct 26, 2013 #4
    Let A be the point where ## \vec{OP} ## meets the plane and P' be the point where P is reflected on the plane:

    finding A:

    ## \vec{OP} = r = \lambda \begin{pmatrix} -3 \\ 2 \\ 1 \end{pmatrix} ## subbing this into the equation of the plane: ## \lambda \begin{pmatrix} -3 \\ 2 \\ 1 \end{pmatrix}. \begin{pmatrix} 1 \\ -2 \\ -2 \end{pmatrix} = 27 ## finding ## \lambda = -3 ##and subbing this into the equation of OP I get A = ## \begin{pmatrix} 9 \\ -6 \\ -3 \end{pmatrix} ##

    Finding P':

    create a line which is normal to the plane and goes through p: ## r = \begin{pmatrix} -3+\lambda \\2-2\lambda \\ 1-2\lambda \end{pmatrix} ## use this to see where it intersects with the plane:

    ## \begin{pmatrix} -3+\lambda \\2-2\lambda \\ 1-2\lambda \end{pmatrix}. \begin{pmatrix} 1 \\ -2 \\ -2 \end{pmatrix} = 27 ## find ## \lambda = 4 ## double this and sub ## \lambda = 8 ## into the equation of the line with direction of the normal of the plane and find that ## P' = \begin{pmatrix} 5 \\ -14 \\ -15 \end{pmatrix} ## now do P' - A and I get ## P' - A = \begin{pmatrix} -4 \\ -8 \\ 18 \end{pmatrix} ## so the ## cos(a,b,c) = \dfrac{-4}{2\sqrt{101}},\dfrac{-8}{2\sqrt{101}},\dfrac{18}{2\sqrt{101}} ##
     
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