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The purpose of this paper (which, in many passages, I take the liberty ofwriting in the first person singular for bringing a personal experience) is toshow how teachers of Physics and Mathematics in Basic Education can teach,through a simple and playful activity, the rules of signs that are related tobasic arithmetic operations involving positive and negative numbers. How? Byusing 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 isphysical and mathematical, this paper provides a succinct review on theformation of images by flat and spherical mirrors since some teachers ofMathematics may (just as a possibility) not know how it happens. Lastly, giventhe interdisciplinary nature of this activity, some additional considerationsare also presented regarding how these teachers can use it to instigate thestudents to observe how Physics and Mathematics, together, are part of everydaylife for all of us.

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