The acute angle between the two lines whose d.r’s are given by the equations  l+m+n=0 and l2+m2−n2=0 is

Given n=-l-ml2+m2--l-m2=0 2lm=0 l=0orm=0 l=0⇒n=-m l,m,n=0,1,-1similarly m=0(0,1,-1)and(1,0,-1)Solve the given two equation for the d.r's of the lines and use cosθ formula