### Definition of perimeter

The word perimeter is borrowed from the latin perimĕtros, which in turn derives from a Greek concept. It refers to the outline of a surface or a plane figure and the calculation of this contour. In other words, in the case of a plane figure, the perimeter is the sum of the lengths of its sides. Thus, the perimeter to calculate the boundary of a surface, which is very useful.
Know the perimeter of a field (or a lot), for example, to establish how much of the material is necessary to close it. Furthermore, the perimeter is an essential data to plan the security of a home or a private condominium.
It is worth mentioning that if the perimeter is the data that is used to calculate the edges of a surface, the area, in turn allows, to know its inside surface. Therefore, the perimeter will show us how can we close a field, while that the area will bring us information on how can we cover such field or even how much fertilizer use.
To calculate the perimeter of a surface, you must know the length of all its sides. For example: a triangle with sides measuring 3 cm, 8 cm and 9 cm, has a perimeter of 20 centimeters.
Sometimes, the perimeter can also know the data discarded on a side. Knowing that a triangle has a perimeter of 15 cm, and two of its sides measure 5 and 2 centimeters, the third side should measure 8 cm. It's a problem of rule of three (or rule of proportionality).