Two Dimensional Figures
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Triangle
The closed figure formed by joining three line segments end-to-end is called a triangle. Each line segment forms a side of the triangle. It is a polygon with least number of sides.

Types of triangles based on sides

Scalene triangle:
A triangle is called a scalene triangle if all the three sides are of unequal length.

Isoceles triangle:
A triangle is called an isosceles triangle if two of its sides are of equal length.

Equilateral triangle:
A triangle is said to be an equilateral triangle if the lengths of all of its sides are equal.

Types of triangles based on angles

Acute-angled triangle
:
If all the angles of a triangle are less than 90°, then the triangle is called an acute-angled triangle.

Right-angled triangle:
If one of the angles in a triangle is a right angle, then the triangle is called a right-angled triangle.

Obtuse-angled triangle:
If one of the angles in a triangle is an obtuse angle, then the triangle is called an obtuse-angled triangle.

Quadrilaterals

Parallelogram
:
A parallelogram is a four-sided figure in which the opposite sides are parallel to each other and are of equal length. In a parallelogram, the diagonals need not be equal in length.

Rectangle:
A rectangle is a type of parallelogram that has opposite sides equal in length and parallel to each other. Its diagonals are equal in length. A rectangle has four right angles.

Square:
A square is a type of parallelogram in which all the four sides are equal in length. Its diagonals are equal in length. A square has four right angles.

Rhombus
In a rhombus, all the sides are equal in length, and the opposite sides are parallel to each other. Its diagonals are not equal in length. Also, the opposite angles are equal to each other.

Trapezium
A trapezium has one pair of sides parallel to each other. The other two sides are not parallel to each other.

Polygon
A polygon is a closed figure with three or more than three sides. Polygons are named based on their sides. A polygon with 3 sides is a triangle. A polygon with four sides is a quadrailteral. A polygon with five sides is a pentagon. A polygon with six sides is called a hexagon. And a polygon with 8 sides is called an octagon.

Summary

Triangle
The closed figure formed by joining three line segments end-to-end is called a triangle. Each line segment forms a side of the triangle. It is a polygon with least number of sides.

Types of triangles based on sides

Scalene triangle:
A triangle is called a scalene triangle if all the three sides are of unequal length.

Isoceles triangle:
A triangle is called an isosceles triangle if two of its sides are of equal length.

Equilateral triangle:
A triangle is said to be an equilateral triangle if the lengths of all of its sides are equal.

Types of triangles based on angles

Acute-angled triangle
:
If all the angles of a triangle are less than 90°, then the triangle is called an acute-angled triangle.

Right-angled triangle:
If one of the angles in a triangle is a right angle, then the triangle is called a right-angled triangle.

Obtuse-angled triangle:
If one of the angles in a triangle is an obtuse angle, then the triangle is called an obtuse-angled triangle.

Quadrilaterals

Parallelogram
:
A parallelogram is a four-sided figure in which the opposite sides are parallel to each other and are of equal length. In a parallelogram, the diagonals need not be equal in length.

Rectangle:
A rectangle is a type of parallelogram that has opposite sides equal in length and parallel to each other. Its diagonals are equal in length. A rectangle has four right angles.

Square:
A square is a type of parallelogram in which all the four sides are equal in length. Its diagonals are equal in length. A square has four right angles.

Rhombus
In a rhombus, all the sides are equal in length, and the opposite sides are parallel to each other. Its diagonals are not equal in length. Also, the opposite angles are equal to each other.

Trapezium
A trapezium has one pair of sides parallel to each other. The other two sides are not parallel to each other.

Polygon
A polygon is a closed figure with three or more than three sides. Polygons are named based on their sides. A polygon with 3 sides is a triangle. A polygon with four sides is a quadrailteral. A polygon with five sides is a pentagon. A polygon with six sides is called a hexagon. And a polygon with 8 sides is called an octagon.

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