Теория:

Introduction:
A parallelogram is a special kind of quadrilateral in which both pairs of opposite sides are parallel.
In the following figure, you can see that ABCD and BCAD
 
parallel.png
The four basic properties of a parallelogram are:
  1. Opposite sides of a parallelogram are equal
  2. Opposite angles of a parallelogram are equal
  3. Diagonals divide the parallelogram into two congruent triangles
  4. Diagonals bisect each other
There are three special types of a parallelogram, they are:
  1. Rectangle
  2. Rhombus
  3. Square
The following figure explains the special types of a parallelogram.
 
Introduction to parallelogram-Venn diagram.png
Important things to know about parallelogram
Area of a parallelogram
The area is the base times the height.
area\(= b × h\)
(\(h\) is at right angles to \(b\))
The Perimeter of a parallelogram
The perimeter is the distance around the edges.
The perimeter is \(2\) times the (base + side length):
Perimeter\(= 2(b+l)\)
Diagonals of a parallelogram
The diagonals of a parallelogram bisect each other.
In other words, the diagonals intersect each other at the half-way point.
In the next chapter, we will explore the properties of a parallelogram.
  
Interesting fact:
A parallelogram where all angles are right angles is a rectangle!