Given, xy + yz = 0

⇒ x(y + z) = 0

⇒ x = 0 and y + z = 0

Clearly, above equations are equation of planes [As they have form Ax + By + Cz + D = 0 form]

Also, x = 0 has normal vector

And y + z = 0 has normal vector

And dot product of these two is

= 0 + 0

= 0

Hence, planes are perpendicular [As the dot product of normal of two perpendicular planes is 0]