If l1, m 1, n1 and l2, m 2, n2 are the direction cosines of two mutually perpendicular
lines, show that the direction cosines of the line perpendicular to both of these
are  
1
2
2
1
1
2
2
1
1
2
2
1
,
,
m n
m n
n l
n l
l m
l m
−
−
−
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 MATHEMATI CS
498
3 Find the angle between the lines whose direction ratios are a, b, c and
b – c, c – a, a – b 4 Find the equation of a line parallel to x-axis and passing through the origin