To determine the equation of the sphere tangent to the plans XOY and \pi:2x-y+2z+5=0 and takes centre on the straight (r)
(r):
x=2
y=-1


Answer:
x^2+y^2+z^2-4x+2y+4z+5=0 and x^2+y^2+z^2-4x+2y-20z+5=0


My idea:
distance from the centre to plan XOY equals distance from the centre to plan \pi. But can not understand where use of the equation straight