lc:sqrt(sum(c[i]**2,i,1,3));
sin(sqrt(x^2+y^2))/sqrt(x^2+y^2), [x,-20,20], [y,-20,20]
cos( (x^2+y^2)/4)/(x^2+y^2+%pi), [x,-5,5], [y,-5,5]
cos((x^2+y^2)/4)/(x^2+y^2+%pi), [x,-5,5], [y,-5,5]
cos(x^2+y^2)/(1+x^2+y^2)
x^3-3xy^2
cos(abs(x)+abs(y))
acos(x)*180/%pi