Basic Geometrical Ideas

- A point determines a location. It is usually denoted by a capital letter.

- A line segment corresponds to the shortest distance between two points. The line segment joining points A and B is denoted by AB . AB and BA denote the same line segment.

- A line is obtained when a line segment like AB is extended on both sides indefinitely; it is denoted by AB or sometimes by a single small letter like l.

- Two distinct lines meeting at a point are called intersecting lines.

- Two lines in a plane are said to be parallel if they do not meet.

- A ray is a portion of line starting at a point and going in one direction endlessly.

- Any drawing (straight or non-straight) done without lifting the pencil may be called a curve.
In this sense, a line is also a curve.

- A simple curve is one that does not cross itself.

- A curve is said to be closed if its ends are joined; otherwise it is said to be open.

- A polygon is a simple closed curve made up of line segments. Here,
(i) The line segments are the sides of the polygon.
(ii) Any two sides with a common end point are adjacent sides.
(iii) The meeting point of a pair of sides is called a vertex.
(iv) The end points of the same side are adjacent vertices.
(v) The join of any two non-adjacent vertices is a diagonal.

- An angle is made up of two rays starting from a common end point.

- Two rays OA OA and OB make ∠AOB (or also called ∠BOA ).

- An angle leads to three divisions of a region:

- On the angle, the interior of the angle and the exterior of the angle.

- A triangle is a three-sided polygon.

- A quadrilateral is a four-sided polygon. (It should be named cyclically). In any quadrilateral
ABCD, AB & DC and AD & BC
 are pairs of opposite sides. ∠A & ∠C and ∠B & ∠D
 are pairs of opposite angles. ∠A is adjacent to ∠B & ∠D ; similar relations exist for other three angles.