2. CHAPTER 2 : POLYGONS II

**Names of Polygons**

Name | Sides | Interior Angle | Exterior Angle |

Triangle (or Trigon) | 3 | 60° | 120° |

Quadrilateral (or Tetragon) | 4 | 90° | 90° |

5 | 108° | 72° | |

6 | 120° | 60° | |

Heptagon (or Septagon) | 7 | 128.571° | 51.429° |

Octagon | 8 | 135° | 45° |

Nonagon (or Enneagon) | 9 | 140° | 30° |

Decagon | 10 | 144° | 36° |

3. CHAPTER 3 : CIRCLES II

**Definitions related to circles:**

**Arc:**A continuous piece of a circle is called an arc. In other words, any portion of the circumference of a circle is called an arc.

**Chord:**A straight line joining any two points on the circumference of a circle is called a chord.

**Circumference:**The perimeter of a circle is called its circumference

**Diameter:**Any straight line drawn through the centre and terminating at both ways by the circumference is called a diameter.

**Origin:**Origin refers to the center of a circle

**Pi ( ):**An approximate value of is 22/7 which is correct to two decimal places. A more accurate value of is 3.14159 which is correct to five decimal places.

**Radius:**The constant distance of every point on the circle from its centre is called the radius of the circle. It is half of the diameter.

**Sector:**A sector is that part of a circle, which lies between an arc and two radii joining the extremities of the centre. The most important sector is a quadrant, which is one-fourth of the circle.

**Tangent of a circle:**It is a line perpendicular to the radius that touches only one point on the circle.

**Circumference of a circle:**= 2 r where is 22/7 or 3.14159