Rectangular call a triangle, the corner in one of tops of which is equal 90 °. The party lying opposite to this corner is called by a hypotenuse, and by the parties, opposite to two acute angles of a triangle, are called legs. If length of a hypotenuse and size of one of acute angles is known, then these data are enough to construct a triangle in, at least, two ways.

Two parties of a triangle forming its right angle are perpendicular the friend the friend, as found reflection in their Greek name ("legs") today used everywhere. Each of these parties is adjoined by about two corners one of which to calculate there is no need (right angle), and another always is sharp and it is possible to calculate its size in several ways.

Construction works and also re-planning of the apartment and preparation for its repair demand not only construction skills, but also knowledge of mathematics, geometry and so forth. So, often happens it is necessary to find an internal corner of a triangle.

Circle - one of the main curves studied in elementary and higher mathematics. The circle, in turn, represents the figure which is in the section of many bodies of rotation. The cylinder and a cone concern those, in particular.