In: Advanced Math
(a) Given that, a,b are two nonzero vectors wose directions are
inclined at 
 .
The vector product of a & b is denoted as
  
and it gives a vector quantity whose modulus value is 
 and the direction is along the perpendicular to both vectors.
=
where 
 denoted as the unit vector perpendicular to both a & b.
(b) The position vectors of
A= 2i+j+3k, B = 4i-j+7k, C = -i-2j-2k , D = 2i-5j+k
then,
the equation of the line AB,

and the equation of line CD,

Any point of the line AB can be written as 
Any point of the line CD can be written as 
No such 
 exists such that, 
= 
Hence, AB, CD are skew.
Now, let the shortest distance line of AB and CD meets AB at E and CD at F.
Let, E= 
and F= 
Then, EF=
which is perpendicular to (2,-2,4) and (3,-3,3)
Then,

and 
which gives 
Then, the common perpendicular vector EF= (-3i-3j+0k)
and the shortest distance d= 
 unit