F.5A.MATHS

magnitude of u = 1.
magnitude of v = 1.
Angle between u and v is (y - x).
u dot v = cosxcosy + sinxsiny = magnitude of u x magnitude of v x cos (y -x).
Therefore, cos(y -x) = cos x cos y + sin x sin y.
Note: cos (y - x) = cos[-(x - y)] = cos(x - y).



2009-09-21 08:14:15 補充：
To be more precise, u dot v = (magnitude of u)(magnitude of v) cos (angle between the 2 vectors) = (1)(1)cos(y -x) = cos(y -x).