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| A polygon is a closed figure made by joining line segments, where each line segment intersects exactly two others. |
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| 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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| A three-sided polygon. The sum of the angles of a triangle is 180 degrees. |
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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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| A four-sided polygon. The sum of the angles of a quadrilateral is 360 degrees. |
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| A four-sided polygon having all right angles. The sum of the angles of a rectangle is 360 degrees. |
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A four-sided polygon having equal-length sides meeting at right angles. The sum of the angles of a square is 360 degrees.
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A four-sided polygon having equal-length sides meeting at right angles. The sum of the angles of a square is 360 degrees.
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A four-sided polygon having equal-length sides meeting at right angles. The sum of the angles of a square is 360 degrees.
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| A four-sided 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 five-sided polygon. The sum of the angles of a pentagon is 540 degrees. |
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| A six-sided polygon. The sum of the angles of a hexagon is 720 degrees. |
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| A seven-sided polygon. The sum of the angles of a heptagon is 900 degrees. |
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| An eight-sided polygon. The sum of the angles of an octagon is 1080 degrees. |
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| A nine-sided polygon. The sum of the angles of a nonagon is 1260 degrees. |
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| A ten-sided polygon. The sum of the angles of a decagon is 1440 degrees. |
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| A circle is the collection of points in a plane that are all the same distance from a fixed point. The fixed point is called the center. A line segment joining the center to any point on the circle is called a radius. |
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| A figure is convex if every line segment drawn between any two points inside the figure lies entirely inside the figure. A figure that is not convex is called a concave figure. |
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