一、设而不求,自行消灭有些题目中,由于某些线段长度未知,且和已知线段没有数量关系,有些学生觉得用到点的坐标无法求解而放弃.其实,要证的结论和这个点的坐标可能无关,故可以巧设辅助量,在向量运算时辅助量会自行消灭.例1如图1,在四棱锥P-ABCD中,PA⊥平面ABCD,四边形ABCD为平行四边形,AB=1,BC=槡2,∠ABC=45°,点E在PC上, First, set but not demand, eliminate some of the problems on their own, because some of the length of the line is unknown, and there is no quantitative relationship with the known line segment, some students feel that using the coordinates of the point can not be solved and give up. In fact, the conclusion of the proof and this The coordinates of the points may be irrelevant, so the auxiliary quantity can be skillfully set, and the auxiliary quantity will self-extinguish in the vector operation. Example 1 In the quadrangular pyramid P-ABCD, the PA ⊥ plane ABCD and the quadrilateral ABCD are parallelograms, AB= 1, BC=槡2, ∠ABC=45°, point E is on the PC,