Originally Posted by

**Jenny20** Question is in the attachment. Please open the attachment. Thank you.

The file is pretty messed up, but I think I got the question.

(I changed the condition on k because the problem statement was not correct. k need not be negative.)

By definition:

where is the angle between the two vectors.

So the problem statement:

means that

which means that = 0 or 180 degrees. Thus the vectors are parallel or antiparallel.

Vectors which are parallel (or antiparallel) are the same vector, up to a rescaling, ie. there exists a scalar k (which just changes the length of the vector) such that . (k is positive if the vectors are parallel and negative if they are antiparallel.)

-Dan