kill(all)$

load(ntrig)$
load(atrig1)$

van=facebook.com/LearningCalculus$

rad(theta):=%pi/180*theta$
sinprod(a,b):=(1/2)*(cos(a-b)-cos(a+b))$
f(x,y):=sinprod(x,y)$ f(x,y)$

a:rad(2235);A:5*%pi/12;

b:rad(1845);B:1*%pi/4;

sinprod(A,B);

fullratsimp(%);

f(x,y) = %;