\[\boxed{\text{643\ (643).}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[2x^{2} - 4x + b = 0\]
\[D = 16 - 4 \cdot 2 \cdot b = 16 - 8b\]
\[x_{1,2} = \frac{4 \pm \sqrt{16 - 8b}}{4}\]
\[1.\ \ b < 2:\]
\[x = \frac{4 \pm \sqrt{16 - 8b}}{4}\]
\[2.\ b = 2:\]
\[x = \frac{4 \pm 0}{4} = 1\]
\[3.\ b > 2\]
\[16 - 8b < 0\ \]
\[нет\ корней.\]