The mass of tasks is on the basis of properties of polyhedrons. Sides of volume figures, as well as concrete points on them, lie in the different planes. If to carry out one of such planes under a certain corner through a parallelepiped, then the part of the plane lying within a polyhedron and dividing it into parts will be its section.

Rectangle - one of the simplest geometrical figures which, having all corners identical and equal 90 °, is a special case of a parallelogram. The piece connecting two tops of a rectangle which do not have the general party is called the diagonal of this polygon. It is possible to calculate diagonal length in several ways depending on the known basic data.

Sections of geometrical figures have various forms. At a parallelepiped the section always represents a rectangle or a square. It has a number of parameters which can be found in the analytical way.

Rectangle call a special case of a quadrangle - the closed geometrical figure made of four pieces which are not lying on one straight line in pairs connecting four tops of this polygon. Distinctive feature of a rectangle are corners in 90 °, lying in each its top. This feature considerably simplifies a problem of finding of length of diagonal of a figure, almost always reducing it to Pythagorean theorem.