Find the equation of the circle which is circumscribed about the triangle whose vertices are A( - 2, 3), b(5, 2) and C(6, - 1). Find the centre and radius of this circle.
The general equation of a circle: $(x-h)^{2}+(y-k)^{2}=r^{2}$
$\ldots(\mathrm{i})$, where $(\mathrm{h}, \mathrm{k})$ is the centre and $\mathrm{r}$ is the radius.
Putting $A(-2,3), B(5,2)$ and $c(6,-1)$ in (i) we get
$h^{2}+k^{2}+4 h-6 k+13=r^{2} \ldots$ (ii)
$h^{2}+k^{2}-10 h-4 k+29=r^{2} \ldots$ (iii)and
$h^{2}+k^{2}-12 h+2 k+37=r^{2} \ldots$ (iv)
subtracting (ii) from (iii) and also from (iv),
$-14 h+2 k+16=0 \Rightarrow-7 h+k+8=0$
$-16 h+8 k+24=0 \Rightarrow-2 h+k+3=0$
Subtracting,
$5 h-5=0 \Rightarrow h=1$
$k=-1$
Centre $=(1,-1)$
Putting these values in (ii) we get, radius
$=\sqrt{1+1+4+6+13}=\sqrt{25}=5$
Equation of the circle is
$(x-1)^{2}+\{y-(-1)\}^{2}=5^{2}$
$(x-1)^{2}+(y+1)^{2}=25$