Class 9 Maths Lines and Angles Angles

Angle: Angle is formed when 2 rays originate from same end point. Rays that forms angle are called arms (AB & AC are the arms in figure below) & the endpoint is called vertex. A is the vertex in figure below.

 

Types of Angles

  1. Acute angle : 0o ∠ Angle ∠ 90o
  2. Right Angle :                      Angle= 90o
  3. Obtuse Angle : 90o ∠ Angle ∠ 180o
  4. Straight Angle :                Angle = 180o
  5. Reflex angle : 180o ∠ Angle ∠ 360o

 

Complimentary & supplementary Angles:

2 angles whose sum is 90o are called Complimentary Angles.

2 Angles whole sum is 180o are called Supplementary Angles.

 

Adjacent Angles: Two angles are said to be adjacent if they have a common vertex, a common arm & their non common arm are on different sides of common arm. In the figure below, ∠ ABC & ∠ DBC are adjacent angles. Ray BD us their common arm & point B is their common vertex. Ray BA & BC are non common arm.

 

Linear Pair of Angles: Supplementary Adjacent angles are called Linear Pair of Angles. Or, two adjacent angles whose sum is 180o are called Linear Pair. Non common arm forms a line. In figure above PQR forms straight line.

Vertically Opposite Angle: Two lines intersect itself each other at some point O, forms 2 pairs of vertically opposite angles. In the figure below ∠ AOC = ∠ BOD   & ∠ AOD =∠ BOC. These are 2 pair of vertically opposite angles.

 

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