**Curve:** A figure formed on a plane surface by joining a number of points without lifting a pencil is called a curve.

**Open Curve:** A curve which does not end at the same starting point or which does not cut itself is called an open curve.

**Closed Curve:** A curve which cut itself or which starts and ends at the same point is called a closed curve.

**Simple Closed Curve:** A closed curve called a simple closed curve which does not intersect itself.

**Polygon:** A polygon is a closed figure bounded by three or more line segments such that each line segment intersects exactly two other points (vertices) .

**Quadrilateral:** A simple closed figure bounded by four line segments is called a quadrilateral, it has four sides i.e., AB, BC, CD and AD and four vertices as A, B, C and D and the sum of all angles of a quadrilateral is 360°

**Parallelogram:** A quadrilateral in which opposite sides are parallel and equal is called parallelogram; written as || gm. and the diagonals of a parallelogram bisect each other.

**Properties:**

- Opposite sides are equal and parallel.
- Opposite angles are equal.
- Diagonals bisect each other.

**Rectangle:** A parallelogram each of whose angle is 90° and diagonals are equal, is called a rectangle.

**Properties:**

- Opposite sides are equal and parallel.
- Each angle is a right angle.
- Diagonals are equal.
- Diagonals bisect each other.

**Square:** A quadrilateral in which all sides and angles are equal, is called a square.

**Properties:**

- All the sides are equal and parallel.
- Each angle is a right angle.
- Diagonals are equal.
- Diagonals bisect each other at a right angle.

**Rhombus:** A parallelogram having all its sides equal, is called a rhombus.

**Properties:**

- All the side are equal.
- Opposite angles are equal.
- Diagonals bisect each other at a right angle.

**Trapezium:** A quadrilateral in which two opposite sides are parallel and the other two opposite sides are non-parallel, is called a trapezium.

If two non-parallel sides of a trapezium are equal, then it is called an isosceles trapezium.

The line segment joining the mid-points of non-parallel sides of a trapezium is called it’s median.

**Kite:** A quadrilateral in which two pairs of adjacent sides are equal, is called a kite.

**Properties:**

Diagonals bisect each other at the right angle.

In the figure, m ∠B = m ∠D, but m∠A ≠ m ∠C

Paper is a very common example of a plane surface. The curve obtained by joining a number of points consecutively without lifting the pencil from the paper is called a plane curve. A circle is a very common example of a plane curve.

A polygon is a simple closed curve formed of only line segments. A triangle is a very common example of a polygon.

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