Incorrect conclusions drawn for plausible looking diagrams

Abstract: In Mathematics is common to make a mistake and therefore a false conclusion arises. In each case it is important to recognize the mistake in order to avoid a similar one in the future. Geometric figures provide decisive help in order to have a strict mathematical proof, but also can easily lead to wrong conclusions without a mathematical proof. In this paper, several incorrect conclusions drawn for plausible looking diagrams are presented, motivated by a well-known faulty model for measuring the length of a segment. Similar models that lead to a contradiction are developed and a model that leads to the correct result is derived. The presented models prove the usefulness of paradoxes and can be implemented in a classroom in order to point out to students the significance of a strict mathematical proof as well as the construction of a correct mathematical model. The geometric nature of the problems provides the opportunity to use a dynamic geometric software.