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


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


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