# Triangle and Tetragon

## Triangle

Regular Triangle
The lengths of the three sides and the angles are the same. The size of each of the three angles is 60 °. It is the only triangle in which the inner center, circumcenter, and center of gravity are all in the same location. All regular triangles are isosceles triangles.

Isosceles Triangle
Both sides have the same length. Therefore, the size of both angles on the other side is the same. The vertical bisector of one side with different lengths becomes the axis of linear symmetry. The inner center, circumcenter, and center of gravity are all on this line.

Right Triangle
One vertex is 90˚. Therefore, the sum of the other two angles is 90 °.

Triangle
It consists of three angles and straight-line segments. When drawn on a plane, the sum of the angles of the three vertexes becomes 180 °.

## Tetragon

Square
The lengths of the sides and the angles are the same. The size of each of the four angles is 90 °. All squares are rectangular. By the Pythagorean theorem, the diagonal length is √2 times longer than the length of one side.

Rectangle
All angles are rectangular with right angles. All rectangles become parallelograms. It is the most commonly seen shapes around us, such as TV screens, books, posters, and windows.

Rhombus
All sides have the same length. The angles facing each other are the same size. The two diagonals bisect each other.

Parallelogram
The two pairs of sides are parallel to each other. The angles facing each other are the same, and the sides facing each other have the same length. The two diagonals bisect each other.

Trapezoid
A pair of sides are parallel to each other.

Tetragon
A polygon consisting of four angles and straight-line segments. When drawn on a plane, the sum of the angles of the four vertices makes 360 °.