Solve $x^2-10x+18=0$, giving the roots in simplest surd form.

$D=100-72=28$, so $x=\frac{10\pm \sqrt{28}}{2}=\frac{10\pm 2\sqrt{7}}{2}=5\pm \sqrt{7}$. Every term of the numerator must be divided by 2: dividing only 10 leaves $5\pm 2\sqrt{7}$, and not dividing at all leaves $10\pm 2\sqrt{7}$.