Chọn hệ trục tọa độ như hình vẽ

Ta có \(O\left( {0\,;\,\,0\,;\,\,0} \right),\,\,A\left( {2\,;\,\,0\,;\,\,0} \right),\,\,B\left( {0\,;\,\,3\,;\,\,0} \right),\,\,C\left( {0\,;\,\,0\,;\,\,4} \right),\,\,D\left( {1\,;\,\,0\,;\,\,2} \right)\)

Và \(E\left( {1\,;\,\,\frac{3}{2}\,;\,\,0} \right),\,\,F\left( {0\,;\,\,\frac{3}{2}\,;\,\,2} \right),\,\,P\left( {2\,;\,\,0\,;\,\,4} \right),\,\,M\left( {2\,;\,\,3\,;\,\,0} \right),N\left( {0\,;\,\,3\,;\,\,4} \right).\)

Khi đó \(\overrightarrow {OM} = \left( {2\,;\,\,3\,;\,\,0} \right),\,\,\overrightarrow {ON} = \left( {0\,;\,\,3\,;\,\,4} \right)\)

\( \Rightarrow \left[ {\overrightarrow {OM} \,;\,\,\overrightarrow {ON} } \right] = \left( {12\,;\,\, - 8\,;\,\,6} \right)\) và \[\overrightarrow {OP} = \left( {2\,;\,\,0\,;\,\,4} \right)\].

Thể tích khối tứ diện \[OMNP\] là: \({V_{O.MNP}} = \frac{1}{6}\left| {\left[ {\overrightarrow {OM} \,;\,\,\overrightarrow {ON} } \right] \cdot \overrightarrow {OP} } \right| = 8.\)