Of all the polygons, the triangle has always been the one that exerted a greater fascination in the mathematicians of the whole world. Being a construction that can not be deformed, it was used very early in the architecture of the most diverse buildings, bridges and monuments. Astronomy used its properties to calculate the distance between stars, which could not be calculated in any other way. In navigation, the triangular (or Latin) sail allowed the Portuguese to leave for the discoveries with ships (the caravel) that sailed against the wind.

The triangles can be classified according to the length of the sides and the amplitude of their internal angles. The following table summarizes these two classifications.

Regarding their sides | Equilateral | All three sides are equal | |

Isosceles | Two sides are equal | ||

Scalene | All sides are different | ||

Regarding their angles | Obtuse | One angle is obtuse | |

Right | One angle is right | ||

Acute | All angles are acute |

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