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Find constants $a ,\ b ,\ c \ $ such that the equation of the circle, $x^2+y^2+ax+by=c$, contains the points $(6,8)$, $(8,4)$, and $(3,9)$.

Use the points to create a system:

$\begin{array}{rcrcrcrcr} 6^2 & + & 8^2 & + & 6a & + & 8b & = & c \\ 8^2 & + & 4^2 & + & 8a & + & 4b & = & c \\ 3^2 & + & 9^2 & + & 3a & + & 9b & = & c \\ \end{array}$

Place it into an augmented matrix and perform row operations to reduce:

$\displaystyle \left[ \begin{array}{rr|rcr} 8 & 6 & c &-& 100 \\ 4 & 8 & c &-& 80 \\ 3 & 9 & c &-& 90 \\ \end{array} \right] $ ($R_2-R_3$)

$\displaystyle \left[ \begin{array}{rr|rcr} 8 & 6 & c &-& 100 \\ 1 & -1 & 10 \\ 3 & 9 & c &-& 90 \\ \end{array} \right] $ ($R_1-R_3$)

$\displaystyle \left[ \begin{array}{rr|rcr} 5 & -3 & -10 \\ 1 & -1 & 10 \\ 3 & 9 & c &-& 90 \\ \end{array} \right] $ ($R_1-3R_2; \frac{1}{2}R_1; -R_2)$

$\displaystyle \left[ \begin{array}{rr|rcr} 1 & 0 & -20 \\ -1 & 1 & -10 \\ 3 & 9 & c &-& 90 \\ \end{array} \right] $ ($R_2+R_1$)

$\displaystyle \left[ \begin{array}{rr|rcr} 1 & 0 & -20 \\ 0 & 1 & -30 \\ 3 & 9 & c &-& 90 \\ \end{array} \right] $ ($R_1-R_1$)

This gives $c$ such that the given circle has a negative radius, but surely that can't be. Where is my error?

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    $\begingroup$ The first column should contain the coefficients of $a$, i.e. $(6,8,3)^T$ and the second column the coefficients of $b$, i.e. $(8,4,9)^T$ or vice versa. $\endgroup$ – thanasissdr Sep 8 '15 at 10:10
  • $\begingroup$ The $c$ in your equation is not the square of the radius. You need to complete the squares on the left-hand side first. What is left on the right-hand side after that is the square of your radius. $\endgroup$ – Arthur Sep 8 '15 at 10:11
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As noted in the comment of thanasissdr, the third row of your starting matrix is wrong. It must be $9,3,c-90$

Anyway, solving correctly the system, you find: $a=-6$,$b=-8$ and $c=0$.

This means that the circle has center $C=(\alpha,\beta)=(-a/2,-b/2)=(3,4)$ and radius $r=\sqrt{\alpha^2+\beta^2+c}=5$.

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As Arthur said, once you have rearranged your initial equation into the form $$(x+\frac{a}{2})^2+(y+\frac{b}{2})^2=m^2$$ then m will be your radius. The fact that c is negative is irrelevant unless you find $m^2$ to be negative, them you should be worried

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