Gọi [tex]x\epsilon (A\cap (B\cup C))[/tex]
[tex]\Rightarrow \left\{\begin{matrix} x\epsilon A & & \\ x\epsilon (B\cap C) & & \end{matrix}\right.\Leftrightarrow \left\{\begin{matrix} x\epsilon A & & & \\ x\epsilon B & & & \\ x\epsilon C & & & \end{matrix}\right.\Leftrightarrow \left\{\begin{matrix} x\epsilon (A\cap B) & & \\ x\epsilon (A\cap C) & & \end{matrix}\right.\Rightarrow x\epsilon ((A\cap B)\cup (A\cap C))\Rightarrow[/tex] ĐPCM