Use the distance formula to calculate the diameter sqrroot[(8-2)^2 + (9-1)^2]
=sqrroot(36 + 64)=sqrroot(100)=10. Then radius is 5.
Use midpoint formula to calculate center [(2+8)/2, (1+9)/2] = (5,5)
Use equation for circle (x-h)^2 + (y-k)^2 = r^2
So, (x-5)^2 + (y-5)^2 = 5^2 or (x-5)^2 + (y-5)^2 = 25