Triangle is made by joining any 3 non-collinear points.

It is a 3 sided polygon.

Polygon

Polygon is a simple closed figure made of line segments

A triangle has

3 vertices − A, B, C

3 sides − AB, BC, AC

3 angles − ∠A, ∠B, ∠C

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Interior and Exterior of a Triangle
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Here,

- P, Q are in interior of ∆ABC
- R is on triangle ∆ABC
- X, Y are in exterior of ∆ABC