Whenever we see any shape whether closed or open do we ask ourselves what are these, what are their properties and the most important where can we use them.

- Here we are going to learn about the answers to such questions.
- A line segment is the most basic shape to be learnt.
- A line segment has its very important property that it has a definite length.

Now when we compare two line segments we can do it in the following three ways:

**a) By Observation**: It says that we can compare two line segments by randomly looking at each them and predict about their lengths.**b) By Tracing**: It says that when we keep two lines over each other such that one of their starting points coincide then we can predict which line segment is larger.**c) By using Ruler and Divider:**It says we can predict the length of the line segments by using rulers and scales. It is the most accurate method to measure the lengths of the line segments.

**Angles**

- The inclination of any line from the ground or horizontal level is called as an angle.
- Angles can be measured with the help of an instrument called as a protractor and on the basis of a measured angle
- The angles can be broadly categorized as

**Right Angle**: When the angle is 90° it is said to be right.**Straight Angle**: When the angle is 180° it is said to be straight.**Acute Angle**: When the angle is less than 90° it is said to be an acute angle.**Obtuse Angle**: When the angle is greater than 90° it is said to be an obtuse angle.**Reflex Angle**: When the angle is greater than straight angle itself which means angle greater than 180° is said to be a reflex angle.- The angle covered in one complete revolution is called as a complete angle. It simply means the angle 360° is called as a complete angle.

The Various types of angles are:

**Perpendicular Lines**

- When two lines intersect each other and the angle so formed is 90°, the lines are said to be perpendicular.
- Thus to draw a perpendicular bisector of any line we make a line at 90° to the initial line.

**Classification of Triangles**

A triangle is a 2D shape formed by three line segments which are joined in such a manner to form a closed shape.

- Triangles can be classified on the basis of sides as

**Scalene Triangle**: Triangles having all the sides of different length is said to be a scalene triangle.**Isosceles Triangle**: Triangles having two sides equal is called as Isosceles triangle.**Equilateral Triangle**: Triangles having all of the sides of equal length are said to be an equilateral triangle.

- Triangle can be classified on the basis of angles as

**Acute Angled Triangle**: Triangles having all the angles acute are said to be acute angled triangle.**Right Angled Triangle**: Triangles having any angle of measurement 90° is said to be a right-angled triangle.**Obtuse Angled Triangle**: Triangles having one angle as obtuse is said to be an obtuse-angled triangle.

**Quadrilaterals**

A polygon having 4 sides is said to be a quadrilateral. Or in simple words, we can say that a quadrilateral is a closed 2D figure having four sides.

- If all the sides of the quadrilateral are equal and all the angles are 90° each then it is said to be a square.
- If the opposite sides are equal and all the angles are 90° each, then it is said to be a rectangle.
- If the opposite sides are equal and parallel and the opposite angles are also equal but not 90°, then it is said to be a parallelogram.
- If all the sides are equal and the opposite angles are also equal but not 90°, then it is said to be a rhombus.

- If only two opposite sides are parallel, then it is said to be a trapezium.

**Polygons**

- Polygons are the basic 2D shapes which are made up of line segments which are arranged in such a manner such that they form a closed figure.
- A polygon is said to be regular if all the sides of the polygon are equal in length as well as the angles are of the same measure.

**The various polygons of different sides are:**

**Three Dimensional Shapes**

- A figure having all the three dimensions that are length, breadth and height are called as a 3D figure.
- A 3D figure has faces, edges and vertex.
- The flat surface of a solid figure is called its face.
- The straight line segments are called as edges.
- The corners formed by the edges when they meet are called as a vertex.

**The Various 3D shapes**

**Recap**

- The inclination of any line from the ground or horizontal level is called as an angle.
- When the angle is 90° it is said to be a right angle.
- When two lines intersect each other and the angle so formed is 90°, the lines are said to be perpendicular.
- Triangles are of three types on the basis of sides a) Scalene b) Isosceles and c) Equilateral.
- On the basis of angles triangles are of three types a) Acute b) Right and c) Obtuse angled triangle.
- A figure having all the three dimensions that are length, breadth and height are called as a 3D figure.
- A 3D figure has faces, edges and vertex.
- A polygon is said to be regular if all the sides of the polygon are equal in length as well as the angles are of the same measure.
- If only two opposite sides are parallel, then it is said to be a trapezium.