Triangle
A polygon is a figure with three or more straight sides. It has to be closed - there cannot be gaps in the figure.Poly- means "many" so a polygon has many sides.
The smallest polygon is a three sided figure called a triangle.
These figures have many sides but they are not polygons because they have a gap - they are not closed.
These are polygons. The one with three sides is a triangle.
Different kinds of triangle
Equilateral
Isoceles
scalene
Right angled
 
Equilateral triangle
This triangle has all three sides the same size. Its angles are also all the same size (60 degrees)
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Isoceles Triangle has 2 sides the same length.
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Scalene triangle has no sides the same size.
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A right angled triangle has one angle that is 90 degrees ( a right angle).
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Geometry: using only compasses and straightedge