Question

The angle between the lines whose direction cosines satisfy the equation l+m+n=0 and l^2=m^2+n^2

Answer 1

1+m+n=0----------(1)
1^2=m^2+n^2-----(2)
solve these equation :-
m+n= -1 - - - - -(***)
1=m^2+n^2 - - - - - - (#)
from equation (#) :- we get

1=(m+n)^2
1= (-1)^2 ------( from *** equation)
1 = 1
hence proved