In the Classroom are a and b. Thus, he may see more clearly the idea of the solution which is to introduce an auxiliary problem whose unknown is y. Finally, working at one right tri angle after the other, he may obtain(see Fig. 1) and hence, eliminating the auxiliary unknown y, The teacher has no reason to interrupt the student if he carries these details torrectly except, possibly, to warn him that he should check each step. Thus, the teacher may ask: x, y, c"Can you see clearly that the triangle with sides is a right triangle?" answer honestly To this question the student may"Yes" but he could be much embarrassed if the teacher not satisfied with the intuitive conviction of the student, should go on asking: is a right tri"But can you prove that this triangle angle?" suppress this question Thus, the teacher should rather unless the class has had a good initiation in solid geome- try. Even in the latter case, there is some danger that the answer to an incidental question may become the main difficulty for the majority of the students. 13. Looking back. Even fairly good students, when they have obtained the solution of the problem and writ ten down neatly the argument, shut their books and look for something else. Doing so, they miss an important and instructive phase of the work. By looking back at the completed solution, by reconsidering and reexamining the result and the path that led to it, they could consoli.