☼ ANGLES  :   The union of  two rays having the same initial point and not lying in one line is called an angle.

Consider two rays   and   meeting at the common initial point O, then these two rays form an angle AOB. The point O is called the vertex of  angle  AOB. The rays   and    forming the angle are called its arms ( or sides ).

The symbol   is used to denote an angle  AOB is written as  AOB.

We can also denote an angle by simply its vertex.

Note  :  (i) If  an angle  AOB is on plane X, then AOB divides ( partitions ) the plane into the following three subsets of  X.

• AOB
• Interior of  AOB
• Exterior of   AOB

(ii) Cross Bar Theorem  : –  If  D is in the interior of  an angle  AOB, then  intersects .

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