It Is one of the most difficult tasks in teaching elementary mathematics in high schools to be both, understandable and correct. Most textbook authors know how to write simply and so to speak popularly, but, on the other hand, there is no doubt that many of them fall short of perfect scientific correctness. It would be ridiculous to try putting in school classes such modern scientific methods as, for example, the formal introduction of real fractions as couples of integral numbers, negative numbers as couples of positive numbers, imaginary numbers as couples of real numbers. However, we are often forced as teachers to meet a mathematical situation where we have to pay attention to two points: first, no conclusion must be gained surreptitously or by tricks, and secondly, when some of the steps within a statement are too difficult to be understood by young people the gaps must not be concealed but, on the contrary, pointed out to the students with the explanation that it is possible to accomplish the proof although we may not do so for the moment.