**Key Concepts**

- Lines
- Classification of Triangles
- Classification of Quadrilaterals

**Lines**

**Introduction: **

**Definition**

**Line:**

A line is a figure formed when two points are connected with minimum distance between them and have no ends on both sides.

A line *AB* is represented by

AB↔

A real-life example of a line

**Line Segment:**

A line segment is part of a line that has a fixed endpoint on both the ends.

A line segment *AB* is represented by AB−

A real-life example of a line segment

**Ray:**

A ray is a part of a line that has only one fixed point, and the other point does not have any end.

A line segment *AB* is represented by AB→

A real-life example of a ray

**Types of lines:**

Basically, lines are of 3 types:

- Parallel lines

- Intersecting lines

- Perpendicular lines

**Parallel lines**

Two lines are said to be parallel lines if they lie in the same plane and never meet.

*Parallel lines*

*Non- Parallel lines*

A real-life example of parallel lines

**Intersecting lines**

If two lines meet at a point, they are said to be intersecting lines.

A real-life example of intersecting lines

Scissors, the two blades of the scissors intersect at each other to make it work effectively.

**Perpendicular lines**

When two lines intersect at an angle of 90^{o }or at a right angle, then they are perpendicular to each other.

A real-life example of perpendicular lines

Clocks are the best examples to show perpendicular lines at 3 O’ clock and 9 O’ clock.

**Classification of Triangles**

**Triangle (Definition)**

A triangle is a closed shape with three angles, three sides and three vertices.

Triangles can be classified as:

- On the basis of angles
- On the basis of measurement of their sides

A real-life example of triangles

**Classification of Quadrilaterals**

**Quadrilateral**

A quadrilateral is a closed two-dimensional shape that has 4 sides, 4 angles, and 4 vertices.

A real-life example of a quadrilateral

**Types of Quadrilaterals:**

Quadrilaterals are classified based on their sides, angles, and diagonals.

A real-life example of types of quadrilaterals

**Exercise:**

- Find out the number of sleeping lines, slanting lines, curved lines, circles, square, triangle and rectangle in the figure below:

- Is it possible to have an obtuse acute triangle? Explain.
- What is true about all quadrilaterals?
- What is the difference between a square, and a rhombus?
- Can a trapezoid have perpendicular sides?
- Classify each triangle by its sides, and then by its angles.
- The blades of an open pair of scissors look like what pair of lines?
- What geometric term could you use to describe the top, and bottom edges of a book?
- How many acute angles are there in an equilateral triangle?
- What kinds of triangles never have right angles?

**What we have learnt:**

- Recognize and define lines, line segments and rays
- Understand parallel and perpendicular lines
- Understand triangles based on angles
- Understand triangles based on sides
- Learn quadrilaterals and classification of quadrilaterals
- Solve questions based on lines, angles, and shapes

**Concept Map:**

**Classification of Lines**