[tex]\bf \textit{equation of a circle}\\\\
(x- h)^2+(y- k)^2= r^2
\qquad
center~~(\stackrel{a}{ h},\stackrel{b}{ k})\qquad \qquad
radius=\stackrel{m}{ r}
\\\\\\
\boxed{(x-a)^2+(y-b)^2=m^2}\\\\
-------------------------------\\\\
(x^2-2ax+a^2)+(y^2-2yb+b^2)=m^2
\\\\\\
x^2-2ax+a^2+y^2-2yb+b^2-m^2=0
\\\\\\
x^2+y^2-2ax-2yb+(a^2+b^2-m^2)=0[/tex]