19. If li,mi,ni; i=1,2,3 denote the direction cosines of three mutually perpendicular vectors in space,prove that AA'=I,where A=[l1 m1 n1 ] [ l2 m2 n2] [l3 m3 n3 ]

19. If li,mi,ni; i=1,2,3 denote the direction cosines of three mutuall

1.	19. If li,mi,ni; i=1,2,3 denote the direction cosines of three mutually perpendicular vectors in space,prove that AA'=I,where A=[l1 m1 n1 ] [ l2 m2 n2] [l3 m3 n3 ]
Answer» 19. If li,mi,ni; i=1,2,3 denote the direction cosines of three mutually perpendicular vectors in space,prove that AA'=I,where A=[l1 m1 n1 ] [ l2 m2 n2] [l3 m3 n3 ]