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Angle between planes

  1. Sep 9, 2012 #1
    1. The problem statement, all variables and given/known data
    Consider two planes with normal vectors n1=<0,-1,-2> and n2=<-2,1,-2> For these planes and angle theta between them


    2. Relevant equations
    n1.n2=||n1|| ||n2|| cos(theta)


    3. The attempt at a solution
    I have gotten my answer for theta to be .729727 radians. Not sure why this isn't right.
     
  2. jcsd
  3. Sep 9, 2012 #2

    vela

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    You need to show your work. We can't tell where you're making mistakes if all you do is tell us your final, wrong answer.
     
  4. Sep 9, 2012 #3
    n1.n2=<0,-1,-2>.<-2,1,-2> = -1-4=-5
    ||n1|| = sqrt(5)
    ||n2|| = sqrt(9)
    pi-cos^-1(-5/sqrt(45)) = .729727
     
  5. Sep 9, 2012 #4

    vela

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    (-2)x(-2) = +4
     
  6. Sep 9, 2012 #5
    n1=<0,-1,2> not -2 sorry
     
  7. Sep 9, 2012 #6

    vela

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    Try using simply the arccosine instead of pi minus the arccosine to get an answer.
     
  8. Sep 9, 2012 #7
    2.41186 is the answer I got and thats not right either.
     
  9. Sep 9, 2012 #8

    vela

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    Well, you're doing the calculation correctly, so there's something else that's wrong.
     
  10. Sep 9, 2012 #9
    Yeah. It's only homework. I'm not a fan of math homework online. Inputs are tricky sometimes. Thanks for the help
     
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