Is the Vector Equation AxB=AxC Only True When A=0 or B=C?

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SUMMARY

The statement "if AxB = AxC then either A=0 or B=C" is false. The equation indicates that either vector A is the zero vector, or the components of vectors B and C that are perpendicular to A are equal. Additionally, the relationship can be expressed as AxB - AxC = Ax(B-C) = 0, which implies that vectors A and (B-C) are parallel. This understanding clarifies the conditions under which the cross product equality holds.

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cp255
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True or False, if AxB = AxC then either A=0 or B=C.

A, B, and C are vectors and I thought this statement would be true. However the answer key says it is not. Why?
 
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This can be explained by following example

1387076046331.jpg
 
Ok that makes sense. Now if the the magnitude of the cross product was not zero then B would have to equal C Right?
 
The magnitude of AxB is equal to the magnitude of A times the magnitude of B times the sine of the angle between the vectors A and B. If the magnitude of C is larger than the magnitude of B, but the sine of the angle between A and C is less, AxC can be equal to AxB.
 
cp255 said:
Ok that makes sense. Now if the the magnitude of the cross product was not zero then B would have to equal C Right?
Not at all. All that AxB=AxC says is that either A=0 (in which case any two vectors B and C satisfy AxB=AxC), or that the components of B and C normal to A are equal.
 
Or that since AxB=AxC is the same as AxB-AxC=Ax(B-C)=0 then A and B-C are parallel.
 
Last edited:

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