The dr’s of two lines are given by a+b+c=0, 2ab+2ac−bc=0. Then the angle between the lines is

a+b+c=0→(1)2ab+2ac−bc=0→(2)⇒2a(b+c)−bc=0,                    a=−(b+c)⇒(b+2c)(2b+c)=0⇒c=−2b or −b2If c=−2b⇒a+b−2b=0⇒a=ba:b:c=1:1:-2If c=−b2 then a+b−b2=0⇒a=−b2⇒a:b:c=−b2:b:−b2=−1:2:−1=1:−2:1If θ is an angle between the lines then cosθ=(1)(1)+(1)(−2)+(−2)(1)1+1+41+4+1=−12⇒θ=2π3 or π3