Open classes are most suitable for teaching something that is not difficult, but particularly useful and must be tested in the college entrance examination. I strongly care that LZ can talk about solid geometry and solve it in three-dimensional coordinate system. Because the textbook has not been revised for a long time, many teachers still use the traditional way of making auxiliary lines to teach solid geometry, which is not only abstract, but also difficult to think of suitable auxiliary lines. It is very practical to solve solid geometry with three-dimensional coordinates, as long as they master the basic vector operation.