Types of Angles A reflex angle is more than 180° A right angle equals 90° 90 ° An acute angle is less than 90° An obtuse angle is more than 90 ° but less.

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Presentation transcript:

Types of Angles A reflex angle is more than 180° A right angle equals 90° 90 ° An acute angle is less than 90° An obtuse angle is more than 90 ° but less than 180°

b b a a c b a d c b a Angle Properties Angles at a point add to 360° a + b + c + d = 360° Angles on a straight line add to 180°. a + b + c = 180° Vertically opposite angles are equal.

d d c c b b a a Parallel Lines Alternate angles are equal. Corresponding angles are equal. f e Same side Interior angles add to 180°. e + f = 180°

Triangles c a b The sum of the angles of a triangle is 180° a + b + c = 180° Equilateral triangles have 3 equal sides and 3 equal angles. a a Isosceles triangles have 2 equal sides and 2 equal angles. 60°

Exterior angles of a triangle The exterior angle of a triangle is equal to the sum of the interior opposite angles. interior opposite angles exterior angle A B C D i.e.  ACD =  ABC +  BAC

20° C A B D E Find  CED = 40° 40°  CDE = 40° 40°  EAB 60° = 120° 120° 55°  CAE = 85° 85°  ACE 35° = 35°  ABE = 20° 20°  AEB = 120° 120° Example

Quadrilaterals p + q + r + s = 360° It can be split into 2 triangles. A quadrilateral has 4 sides. The sum of the angles of a quadrilateral = 2 x 180° = 360°. p q r s

Interior angles of a polygon with n sides add to (n – 2)180°. p q r s t p + q + r + s + t = 540° Interior angles of polygons The sum of the interior angles of any pentagon = 3 x 180° = 540° A pentagon has 5 sides. It can be split into 3 triangles.

b Exterior angles of a polygon Exterior angles of a polygon add to 360°. At each vertex:interior angle + exterior angle = 180° a c e a + b + c + d + e = 360° d

Regular Polygons No. of sidesNameAngle SumInterior Angle 3triangle quadrilateral 180° 60° 360° 90° pentagon 540° 108° hexagon 720° 120° heptagon 900° 129° octagon 1080° 135° nonagon 1260° 140° decagon 1440° 144° What is the exterior angle of each regular polygon? Is the total 360°in each case? 360 / n yes

Find  ABC = 120° 120° Example  ADC = 60° 60°  BAC = 30° 30°  CAD = 30° 30° ABCDE is a regular hexagon with centre O. C A B D E F O  ACD  ODE  EOD = 90° = 60° 60° = 60° 60°