**THEORETICAL GEOMETRY**

The word “geometry” is derived from the combination of two Greek words “geo” and “metron”. The word “geo” means “earth” and “metron” means “measurement”. Thus the subject of “earth measurement” was originally named as “geometry”.

**Basic Geometrical terms**

Point: A

**point**is used to represent a position in space. In practice, we put a small dot on a paper or on a black board to indicate a point. But theoretically, a point has no size or shape. A point can also be understood as the position where two lines intersect each other.**Line:**a

**line**ise the set of points lying at the intersection of two planes.

**Plane**: a

**plane**is a surface extending infinitely in all directions such that all points lying on the line joining any two points on the surface lie on the surface itself.

Points are denoted by capital letters such as

*A*,*B*,*C*and*D*. If*A*and*B*are two points on a line, then the line is denoted by writing↔*AB*and is read as ‘*the line AB*’.The line extends infinitely in two directions

**Line segment :**If

*A*and

*B*are two points on a line, then the portion of the line between

*A*and

*B,*including the points

*A*and

*B,*is called the

**line segment**between

*A*and

*B*and is denoted by the symbol

*AB*. The

**length**of

*AB*is denoted simply by writing

*AB*

**Ray:**A

**ray**is the portion of a line starting from a point on the line extending in one of the two directions of the line. The starting point is called the

**initial point**of the ray.

Two rays →

*AB*and having the common initial point

*A*is said to form an

**angle**at

*A*. The point

*A*is called the

**vertex**of the angle. We shall denote the angle as <

*BAC*or<CAB . Here the segments

*AB*and

*AC*are called a pair of

**arms**of the angle.

Angles are measured by a unit called

**degree.**

**Supplementary angles**If the sum of the measures of two adjacent angles is 180ยบ, then the two angles are called

**supplementary angles**

**Axioms and Theorems on lines**

Property 1: Given any two points on a plane, there is one and only one line containing them. From the above property, we observe that

(i) Two distinct points in a plane determine a unique line. If

*X*and*Y*are any two points on a line, the line*XY (*↔)is denoted simply as the line*XY*, omitting ‘↔’.(ii) Three or more points are called collinear points if they all lie on the same line.

(iii) Three or more points are called non-collinear points if at least one of them does not lie on the line passing through two of the points.

Property 2: Two distinct lines cannot have more than one point in common.

If two distinct lines have a common point, then the lines are called intersecting lines.

If two distinct lines in a plane have no point in common, then the two lines are called non-intersecting lines.

Two non-intersecting lines are called parallel lines.

Property 3: Given a line and a point not on it, there is one and only one line that passes through the given

point and is parallel to the given line.

If three or more lines pass through the same point, then the lines are called concurrent lines.

Property 4: If two lines intersect, then the vertically opposite angles are equal.

Property 5: If a transversal intersects two parallel lines, then any pair of corresponding angles are equal

Theorem 1: If a transversal intersects two parallel lines, then any pair of alternate interior angles are equal.

Theorem 2: If a transversal intersects two parallel lines, then any pair of alternate (interior or exterior) angles

are equal.

Theorem 3: If a transversal intersects two parallel lines, then any pair of interior angles are supplementary.

Property 6: If a transversal cuts two lines such that a pair of alternate angles are equal, then the lines are

parallel.

Theorem 4: If a transversal intersects two lines such that a pair of corresponding angles are equal, then the

lines are parallel.

Theorem 5: If a transversal cuts two lines such that a pair of interior angles on the same side of the

transversal are supplementary, then the lines are parallel.

Theorem 7: If a side of a triangle is produced, then the exterior angle so formed is equal to the sum of the

interior opposite angles.

Property 7: The sum of any two sides of a triangle is greater than the third side.( Note: We observe that in

any triangle

*ABC*,(i)

*AB*<*BC + CA*(ii)*BC*<*CA + AB*(iii)*CA*<*AB + BC.*These inequalities are called triangle inequalities.)

Property 8: In any triangle, the largest side has the greatest angle opposite to it.

Property 9: If any two sides and the included angle of one triangle are equal to any two sides and the

included angle of another triangle, then the two triangles are congruent.

Note: The above property is known as Side-Angle-Side criterion or simply

*SAS*criterion for congruence oftriangles.

Theorem 8: Two triangles are congruent if any two angles and the included side of one triangle are

equal to the two angles and the included side of the other triangle.

Note: The above criterion is known as Angle-Side-Angle criterion or simply as ASA criterion. We observe that in this criterion, a side and the angles on this side of one triangle should correspond to a side and the angles on it of another triangle for congruency.

Theorem 9: Two triangles are congruent if any two angles and a side of one triangle are equal to the two angles and the corresponding side of the other triangle.

Note: The above criterion for congruency is known as the Angle-Angle-Side or AAS Criterion.

Theorem 11: Two right triangles are congruent if the hypotenuse and a side of one triangle are respectively equal to the hypotenuse and a side of the other triangle.

Note: The above criterion is known as the Right-Hypotenuse-Side or RHS criterion for congruence of right triangles.

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