Question 892696
Standard equation for a circle centered at a point (h,k) is {{{highlight_green((x-h)^2+(y-k)^2=r^2)}}}


Note that diameter {{{d=2r}}}.


The first circle centered at the origin is {{{highlight(x^2+y^2=5^2)}}}.


Your next two circles work this way.
The diameter of 20 means that r=10, so this circle, centered at origin is {{{highlight(x^2+y^2=100)}}}.


The last circle of diamter 15 has {{{r=7&1/2}}}, and based on its given center,
{{{highlight((x-1)^2+(y-2)^2=(15/2)^2)}}}
or
{{{highlight((x-1)^2+(y-2)^2=225/4)}}}