find the angle formed by the intersection of each pair of lines
so the Cartesian coordiantes must be (2,3,-1) and (1,2,0) respectively
therefore the angle between is the dot product/magnitude of line 1x magnitude of line two
the book says
William, You've been around long enough to know two things:
1) Folks who can help are volunteers who wander by when they can. If you need an instant answer, you may need to pay for it in some other venue.
2) Many do not even look at posts that are already answered. When you comment on your own post, it looks answered and is much less likely to be answered by anyone else - EVER.
Perhaps the second equation should have a +2z in it?
What do you mean "cartesian coordinates"? Do you mean "direction numbers" or maybe something else?