# The Citardauq Formula

This is interesting — if ax2 + bx + c = 0, then

$\displaystyle x = \frac{2c}{-b\mp \sqrt{b^{2} - 4ac}}.$

It’s called the citardauq formula — citardauq is quadratic spelled backward. It’s explained above by the inimitable James Tanton; here’s a Facebook discussion; and see the second answer here regarding questions of numerical instability.