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Angle between planes

  • Thread starter Colts
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  • #1
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Homework Statement


Consider two planes with normal vectors n1=<0,-1,-2> and n2=<-2,1,-2> For these planes and angle theta between them


Homework Equations


n1.n2=||n1|| ||n2|| cos(theta)


The Attempt at a Solution


I have gotten my answer for theta to be .729727 radians. Not sure why this isn't right.
 

Answers and Replies

  • #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.
 
  • #3
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n1.n2=<0,-1,-2>.<-2,1,-2> = -1-4=-5
||n1|| = sqrt(5)
||n2|| = sqrt(9)
pi-cos^-1(-5/sqrt(45)) = .729727
 
  • #4
vela
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(-2)x(-2) = +4
 
  • #5
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n1=<0,-1,2> not -2 sorry
 
  • #6
vela
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Try using simply the arccosine instead of pi minus the arccosine to get an answer.
 
  • #7
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2.41186 is the answer I got and thats not right either.
 
  • #8
vela
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Well, you're doing the calculation correctly, so there's something else that's wrong.
 
  • #9
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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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