Angular distance
¥
D
A
(x, y)
= angle (x, y)
¥
0
£
D
A
(x, y)
£
p
¥
if 0 then x & y colinear
Ñ
same idea
¥
if
p
/2 then nothing in common
¥
if
p
then D
A
(x, -x) with -x
Ñ
anti-idea
of x
a
x
Õ
y
x