A regular polygon is a polygon whose sides are all the same length, and whose angles are all the same. The sum of the angles of a polygon with n sides, where n is 3 or more, is 180° × (n  2) degrees.
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The following are examples of regular polygons:
Examples:
The following are not examples of regular polygons:
1) The vertex of an angle is the point where the two rays that form the angle intersect.
2) The vertices of a polygon are the points where its sides intersect.
A threesided polygon. The sum of the angles of a triangle is 180 degrees.
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A triangle having all three sides of equal length. The angles of an equilateral triangle all measure 60 degrees.
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A triangle having two sides of equal length.
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A triangle having three sides of different lengths.
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A triangle having three acute angles.
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A triangle having an obtuse angle. One of the angles of the triangle measures more than 90 degrees.
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A triangle having a right angle. One of the angles of the triangle measures 90 degrees. The side opposite the right angle is called the hypotenuse. The two sides that form the right angle are called the legs. A right triangle has the special property that the sum of the squares of the lengths of the legs equals the square of the length of the hypotenuse. This is known as the Pythagorean Theorem.
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Example:
For the right triangle above, the lengths of the legs are A and B, and the hypotenuse has length C. Using the Pythagorean Theorem, we know that A^{2} + B^{2} = C^{2}.
Example:
In the right triangle above, the hypotenuse has length 5, and we see that 3^{2} + 4^{2} = 5^{2} according to the Pythagorean Theorem.
A foursided polygon. The sum of the angles of a quadrilateral is 360 degrees.
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A foursided polygon having all right angles. The sum of the angles of a rectangle is 360 degrees.
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A foursided polygon having equallength sides meeting at right angles. The sum of the angles of a square is 360 degrees.
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A foursided polygon with two pairs of parallel sides. The sum of the angles of a parallelogram is 360 degrees.
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A foursided polygon having all four sides of equal length. The sum of the angles of a rhombus is 360 degrees.
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A foursided polygon having exactly one pair of parallel sides. The two sides that are parallel are called the bases of the trapezoid. The sum of the angles of a trapezoid is 360 degrees.
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A fivesided polygon. The sum of the angles of a pentagon is 540 degrees.
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A regular pentagon:

An irregular pentagon:

A sixsided polygon. The sum of the angles of a hexagon is 720 degrees.
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A regular hexagon:

An irregular hexagon:

A sevensided polygon. The sum of the angles of a heptagon is 900 degrees.
Examples:
A regular heptagon:

An irregular heptagon:

An eightsided polygon. The sum of the angles of an octagon is 1080 degrees.
Examples:
A regular octagon:

An irregular octagon:

A ninesided polygon. The sum of the angles of a nonagon is 1260 degrees.
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A regular nonagon:

An irregular nonagon:

A tensided polygon. The sum of the angles of a decagon is 1440 degrees.
Examples:
A regular decagon:

An irregular decagon:
