Given definition is not a math. An angle is the union of two non collinear rays with a common endpoint is called vertex of the angle and the rays are called the sides.
What is collinear and non collinear rays ?
Non collinear point are three or more points that are not contained on the same time. collinear point lie on the same line. A From this we can define angles. non collinear rays that share the same endpoint form an angle.
You can see by figure
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