Find the angle between the two lines whose direction ratios are:
a, b, c and (b – c), (c – a), (a – b).
The angle between the two lines is given by
cos
= 
where R1 an R2 denote the vectors with the direction ratios,
So, here we have,
R1 = ai + bj + ck and R2 = (b-c)i +(c-a)j + (a-b)k
cos
= 
cos
Hence, 
1