$A=(x_1,y_1,z_1)$ and $B=(x_2,y_2,z_2)$ are two points. If $(l,m,n)$ are the d.c's of CD and $l(x_2-x_1)+m(y_2-y_1)+n(z_2-z_1)=0$ then  the cosine of the angle between the lines AB and CD is