**Toolbox:**

- Solve the two equations to get the d.r of both the lines $ d_1, d_2$
- Prove $ d_1.d_2=0$ for right angle.

Step 1

$ 2l+2m=n$ ...............(1) put n in

$ mn+nl+lm=0$......(2)

Step 2

$ 2lm+2m^2+2l^2+2lm+lm=0$

Step 3

$ 2m^2+5lm+2l^2=0 $

Step 4

factorise and get

$ (m+2l)(2m+l)=0$

$ \Rightarrow m= -2l\: or \: m=\large\frac{-l}{2}$

Step 5

Put $ l=1\: then \: d_1=(1, -2, -2)\: and \: d_2 ( 1, \large\frac{-1}{2}, 1)$

Step 6

$ d_1.d_2=0$