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A decagon is a 10-sided polygon. For a regular decagon, you must make sure that each side and angle is of even measurement, which is very difficult to draw freehand. To determine the exact measurement of angles of any polygon, remember that all angles of a polygon most total 360 degrees. Therefore the angle measurement would be 360 divided by the number of sides. In this case, each angle of a decagon would equal 36 degrees.
Use a ruler and pencil to draw a straight line of any length. Draw this line lightly as a construction line.
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Mark a point on the line, which will act as your first vertex, and label it as point A. Use the protractor to measure and make a point at 36 degrees from A - this will be point B. Draw a line from point A to point B.
Complete the triangle shape from the two lines previously drawn. Measure and draw the remaining side with a ruler, from B to the end of the line - a new point that will be called C; this third side will constitute one side of the decagon.
Measure 36 degrees again out from the AB line, and repeat until you have completed the decagon. The completed shape, if done correctly, will have 10 equal sides, each 36-degree angles.
Use the pentagon shape as an alternative method. Lightly draw and measure a regular pentagon. Lightly draw another identical pentagon directly over the original, rotated 180 degrees. Draw a line between each point of both pentagons to get a decagon shape. Erase the pentagon shapes underneath.