# How do you identify the center and the radius of the circle with equation (x - 3)^2 + (y + 1)^2 = 81?

Jun 11, 2016

centre (3 ,-1) , r=9

#### Explanation:

The standard form of the equation of a circle is.

$\textcolor{red}{| \overline{\underline{\textcolor{w h i t e}{\frac{a}{a}} \textcolor{b l a c k}{{\left(x - a\right)}^{2} + {\left(y - b\right)}^{2} = {r}^{2}} \textcolor{w h i t e}{\frac{a}{a}} |}}}$
where (a ,b) are the coordinates of the centre and r , the radius.

For the given equation: a = 3 , b = -1 and r = 9

hence centre = (3 ,-1) and radius = 9