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Semi math question

  • Thread starter philo51
  • Start date
  • #1
10
0
A(vec)=(2,1,-4) B(vec)=(-3,0,1) C(vec)=(-1,-1,2)

whats the angle between Avec and Bvec in rads.

i know the formula

A(vec)*B(vec)= abs(A)*abs(B)*cos(theta)

A(vec)*B(vec)=-10

so why wont cos^-1(-10/(21)(10)) work?
 

Answers and Replies

  • #2
quasar987
Science Advisor
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absA = [itex]\sqrt{21}[/itex], not 21
 
  • #3
10
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that still isnt right?
 
  • #4
HallsofIvy
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What still isn't right?

You were told that the length of vector A is [tex]\sqrt{21}[/tex], not 21.

Did it then occur to you that the length of vector B should be [tex]\sqrt{10}[/tex], not 10?

Try [tex]\theta= cos^{-1} \frac{-10}{\sqrt{210}}[/tex].
 
  • #5
10
0
no i realized that i just multiplied weird.. sorry thanks!
 

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