given that |a|=|c|=1 and |b|=4
and the angle between b and c is
For each of these cases find the cosine of the angle between a and c.
I don't know where to begin
Secondly, you should work out that , and so that means the second proposition makes sense - the vectors are in the same direction. That is to say, they only differ by a scalar.
What you now want to do is prove that (why?). So, work out what is, and you will be done (why?)! (You will need the information you gleamed from my first line here).
Does that make sense?