Find the angle between the lines whose direction cosines are given by the equations.
(i) l + 2m +3n = 0 and 3lm – 4ln + mn = 0
(ii) l + m + n = 0 and l2 + m2 – n2 = 0
The vectors □(→┬a ) and □(→┬b ) are non-collinear. Find for what value of x, the vectors □(→┬c )=(x-2) □(→┬a )+□(→┬b ) and □(→┬d ) =(2x+1) □(→┬a )- □(→┬b ) are collinear.