The angle between the lines whose direction cosines are given by the equation s l^2+m^2+n^2÷0, l+m+n=0 is .

angle is 120 degrees.

substitute l from equation two in equation 1 you will get an equation in terms of m and n on solving you get two relations between m and n using one at a time to find l you will get two relations between l,m and l. these will be the direction ratios convert them into direction cosines and find the angle