I have a family of spheres
Sa = (x-a)^2 + (y-2a)^2 +z^2 = 81

determine the range of values of a, for which Sa has more than one intersection with the XZ plane (y-axis, y=0). How do i solve this?

Question

I have a family of spheres
Sa = (x-a)^2 + (y-2a)^2 +z^2 = 81

determine the range of values of a, for which Sa has more than one intersection with the XZ plane (y-axis, y=0). How do i solve this?

anonymous · Answer

the answers are supposed to be -9/2 <a < 9/2 but i need the working out for this :)

anonymous · Answer

i guess it buz of the 2's out there 
give me the formula for this
i will try

anonymous · Answer

here is the full question as given

anonymous · Answer

What does the equation tell us about the centre and radius of a sphere?

anonymous · Answer

radius is 9 center is (a,2a,0)

anonymous · Answer

right?

anonymous · Answer

i think i got it :P

anonymous · Answer

Correct :)

anonymous · Answer

Oh, nice!

anonymous · Answer

I'll show you what i thought about it then you can tell me if its right|dw:1395582227452:dw|