Circles

General equation of a circle:

Given that the centre of the circle is point (a,b), with radius r, equation of circle is:

$\left(x-a\right)^2+\left(y-b\right)^2=r^2$

Equation for a tangent of a circle:

When the equation for the circle is $x^2+y^2=r^2$ , tangent to the circle at point (a,b) is

ax+by=r²

To find the equation for the tangent to circle $\left(x-a\right)^2+\left(y-b\right)^2=r^2$ at point (s,t),

from the general equation for a line,