\[По\ теореме\ Пифагора:\]
\[AD^{2} + CD^{2} = AC^{2}\]
\[2^{2} + 2^{2} = AC^{2}\]
\[4 + 4 = AC^{2}\]
\[AC^{2} = 8.\]
\[AB² + AC^{2} = BC^{2}\]
\[2^{2} + 8 = BC^{2}\]
\[4 + 8 = BC^{2}\]
\[BC^{2} = 12 \Longrightarrow BC = \sqrt{12} = 2\sqrt{3}.\]
\[Ответ:BC = 2\sqrt{3}.\]