Find Angle Between Avec and Bvec: Rads Formula Explained

  • Thread starter philo51
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In summary, the conversation is about finding the angle between vectors A and B, using the formula A(vec)*B(vec)= abs(A)*abs(B)*cos(theta). The length of vector A is \sqrt{21} and the length of vector B is \sqrt{10}, not 21 and 10 respectively. The correct formula to use is \theta= cos^{-1} \frac{-10}{\sqrt{210}}. The conversation ends with the person realizing their mistake and thanking the expert for the help.
  • #1
philo51
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 won't cos^-1(-10/(21)(10)) work?
 
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  • #2
absA = [itex]\sqrt{21}[/itex], not 21
 
  • #3
that still isn't right?
 
  • #4
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
no i realized that i just multiplied weird.. sorry thanks!
 

What is the formula for finding the angle between vectors A and B using radians?

The formula for finding the angle between vectors A and B using radians is θ = arccos((A·B) / (|A|·|B|)), where θ is the angle between the two vectors, A·B is the dot product of vectors A and B, and |A| and |B| are the magnitudes of vectors A and B, respectively.

How do I know if the angle between vectors A and B is in radians or degrees?

The angle between vectors A and B is typically given in radians if the formula used is θ = arccos((A·B) / (|A|·|B|)). However, if the formula used is θ = cos-1((A·B) / (|A|·|B|)), then the angle will be in degrees.

Can the angle between vectors A and B be greater than 180 degrees?

No, the angle between vectors A and B cannot be greater than 180 degrees. This is because the range of arccosine (or cos-1) function is limited to 0 to 180 degrees, or 0 to π radians.

What does the dot product of vectors A and B represent?

The dot product of vectors A and B represents the magnitude of vector A multiplied by the magnitude of the component of vector B that is parallel to vector A. This can also be interpreted as the projection of vector B onto vector A.

Can I use this formula to find the angle between more than two vectors?

Yes, this formula can be extended to find the angle between multiple vectors. To do this, you can use the dot product of the first two vectors and then continue multiplying by the cosine of the angle between each subsequent vector and the resulting vector. However, this method may become more complicated and it is often easier to visualize and solve for the angles using other methods, such as drawing a diagram or using trigonometric identities.

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