The parallelogram is a quadrangle which opposite sides are parallel. The straight lines connecting it opposite corners are called diagonals. Their length depends not only on lengths of the parties of a figure, but also on sizes of corners in tops of this polygon therefore without knowledge at least of one of corners it is possible to calculate lengths of diagonals only in exceptional cases. Those are special cases of a parallelogram - a square and a rectangle.

The square represents the correct quadrangle or a rhombus at which all parties are equal and form among themselves corners in 90 degrees. Square diagonal - a piece which connects among themselves two opposite corners of a square. To find square diagonal rather easily

Parallelepiped is called the prism which basis the parallelogram forms. Parallelograms of which the parallelepiped is made are called its sides, their parties — edges, and tops of parallelograms — parallelepiped tops.

Numerical expressions are formed from numbers, signs of arithmetic actions and brackets. If at such expression there are variables, it will be called algebraic. Expression in which the variable contains under signs of trigonometrical functions is trigonometrical. Tasks on determination of values of numerical, trigonometrical, algebraic expressions often meet in a school course of mathematics.