Find an equation of the largest sphere with center (8, 2, 9) that is contained in the first octant.

The point (8,2,9) is in the first octant.
The closest distance from the coordinate planes is the value y=2, which will therefore be the maximum radius of the required sphere.

The standard equation of a sphere with radius r and centre (u,v,w) is given by
(x-u)^2+(y-v)^2+(z-w)^2=r^2

Substituting r=2, u=8, v=2, w=9, we get the equation of the required sphere
(x-8)^2+(y-2)^2+(w-9)^2=2^2