问题　四面体ABCD中 ,点P、Q、R分别是面 ABC、 ACD、 BCD内的一点 ,求作一个截面 ,使其过P、Q、R三点 .作法及说明 :如图 (1 )、(2 ) .1 作直线CP交AB于E ,直线CQ交AD于F .　　 2 若直线EF与BD相交 ,设交点为K ,如图 1 ,连CK ,作直线PQ交CK于L ,再作直线LR交BC、CD分别于M Problem In the tetrahedron ABCD, the points P, Q, and R are points in the planes ABC, ACD, and BCD, respectively, for a cross-section so that they pass P, Q, and R. The method and description are as follows: (1) (2) .1 For the straight line CP cross AB in E, straight line CQ cross AD in F. 2 If the line EF and BD intersect, set the intersection point to K, as shown in Figure 1, even CK, for the straight line PQ cross CK in L, and then for Straight line LR cross BC, CD respectively in M