Find the product of the following monomial:kl,lm,km,klm
If l1,m1,n1,l2,m2,n2 and l3,m3,n3 are the direction cosines of three mutually perpendicular lines,then prove that the line whose direction cosines are proportional to l1+l2+l3,m1+m2+m3 and n1+n2+n3 makes equal angles with them.