kill(all);

van=facebook.com/LearningCalculus$

alpha(a,b,c,d,theta):=arccos((c^2+d^2-(a^2+b^2-2*a*b*cos(theta))/(2*c*d)));

trigsimp(alpha(a,b,c,d,theta));

alpha(a,b,c,d,110/180*pi);

float(cos(float((11*pi)/18)));