Given parallel lines are l (x + y) + p = 0 and l (x + y) – r = 0.

⇒ lx + ly + p = 0 and lx + ly – r = 0

Distance (d) between parallel lines Ax + By + C₁ = 0 and Ax + By + C₂ = 0 is given by

Here A = l, B = l, C₁ = p, C₂ = -r

Distance between parallel lines is

∴