In the space the equation by + cz + d = 0 represents a plane perpendicular to the plane

YOZ ZOX XOY Z = k

YOZ

The equation of YOZ-plane is x = 0 Clearly, planes by + cz + d = 0 and x = 0 are perpendicular as $1 ×0+0×b + 0 × c = 0 $