Basic geometrical ideas(2D)

Triangle 

• A triangle is simple closed figure made up of three line segments.

• There are three angles in a triangle.

• The point of intersection of two adjacent sides of a triangle is called a vertex. There are 3 vertices in a triangle.

• A line segment joining a vertex to the mid- point of the side opposite to the vertex is called a median of the triangle. 

• An altitude of a triangle is the perpendicular drawn from a vertex to the opposite side( produce if necessary).

Quadrilateral

• The figure made up of the four line segments is called the quadrilateral.

• There are four angles in a quadrilateral.

• A line segment joining two non-consecutive vertices is called a diagonal.

• Two sides of a quadrilateral are said to be the adjacent sides of the quadrilateral, if they have a common end point.

• Two sides of a quadrilateral are said to be opposite sides of the quadrilateral, if they are not adjacent sides.