dot product of two vectors A and B is ABcosa where a is angle between them.why???????????????

Dear anuj,

Let's say we have two vectors, A and B, as shown to the left.  The values |A| and |B| represent the lengths of vectors A and B, respectively, and Θ is the angle between the two vectors.  The dot product of vectors A and B will have the following relationship to these values:

A · B = |A| * |B| * cos(Θ)

That is to say, the dot product of two vectors will be equal to the cosine of the angle between the vectors, times the lengths of each of the vectors.