Uma hist\'oria e o uso de espelhos no aprendizado das regras de sinais envolvendo n\'umeros negativos

The purpose of this paper (which, in many passages, I take the liberty of writing in the first person singular for bringing a personal experience) is to show how teachers of Physics and Mathematics in Basic Education can teach, through a simple and playful activity, the rules of signs that are related to basic arithmetic operations involving positive and negative numbers. How? By using a ruler, or any other object that can be interpreted as a number line, and a flat mirror. By the way, as the justification of why this method works is physical and mathematical, this paper provides a succinct review on the formation of images by flat and spherical mirrors since some teachers of Mathematics may (just as a possibility) not know how it happens. Lastly, given the interdisciplinary nature of this activity, some additional considerations are also presented regarding how these teachers can use it to instigate the students to observe how Physics and Mathematics, together, are part of everyday life for all of us.
Comment: 35 pages, 16 figures (this paper was purposely/exceptionally written in portuguese)