Thread: Angle Between Two Vectors

Greetings,

I would like to ask whether the cosine of the angle between two vectors can be derived from the knowledge of directional cosines. If that is the case, how can I prove the statement? I am attaching a scanned page from the book Vector and Tensor Analysis by Hay, which illustrates the concept beginning with the phrase, "By a formula from of analytic geometry..." and writes the formula, around which I have put a red box, below this statement. I have put a red rectangular box to emphasize my point.

Thanks

The theorem in the red box is pretty much a restatement of (7.1). Not particularly helpful.

To find , draw vectors and , along with their difference, , forming a triangle. Note that the dot product obeys the distributive property, that is

. However by the law of cosines,



Set these two expressions equal, cancel stuff, and you will obtain , or

Thanks, you are right, but I think you emphasize that no such formula exists in analytical geometry if I am not mistaken. However, I still wonder why the author wrote, "By a formula of analytical geometry..." do you have an explanation?