I picked up A Course in Pure Mathematics, which itself isn't exactly high school material, but I have a very simple question; simple enough that I felt it would be silly to post it in the college forum.

The expression I'm wondering about can be seen here:
My question is, what exactly does the dot mean? The next sentence reads "if we take A_0A_1 as our unit of length, and write A_0A_1 = 1, then we have"

Thanks in advance for any help.

A_0A_r is just the vector from the origin to the circumference of a circle with radius r .

example : a_0=(0,0) and A_r=(3,4)
A_0,A_r = (3-0,4-0)
If we normalize this vector we get A_0A_1=(3/5,4/5)
where 5 =sqrt(3^2+4^2)=r

Or , A_0A_1 =(3/r,4/r)
Which means that A_0A_r= r*(3/r,4/r) where r is a scalar product