- Write the equation of a circle given its graph, or its center and radius
If you know the center [latex](h,k)[/latex] and radius [latex]r[/latex], the equation is [latex](x-h)^{2}+(y-k)^{2}=r^{2}.[/latex]
Write the equation of the circle with center [latex](2,-5)[/latex] and radius [latex]4[/latex].
[latex]\begin{align} (x-h)^{2}+(y-k)^{2}&=r^{2}\\ (x-2)^{2}+(y+5)^{2}&=4^{2}\\ (x-2)^{2}+(y+5)^{2}&=16 \end{align}[/latex]