The Decagon will be introduced to your student around the fifth grade simply for the purpose of identification, and for no other reason.
I, however think all kids, should also be taught the connections between the different regular polygons.
You are watching: How do you draw a decagon
These connections mainly are for finding the area and angles of a regular polygon. In a four sided regular polygon (square)the degree value of its internal angles are found byDividing 360 by 4 = 90(This gives the values of the 4 angles at the center of the square)Subtract 90 from 180 to get the value of the internal angles. 180 - 90 = 90
in a 5 sided regular polygon the degrees are found byDividing 360 by 5 = 72 (This gives the values of the 5 angles at the center of the square)Subtract 72 from 180 to get the value of the internal angles. 180-72=108
The pattern is simply 360 divided by the number of sides of the regular polygon to get the angles at the center. Then subtract your answer from 180 to get the value of the internal angles.
So even if your child is asked about a 20 sided polygon - they should have by now, developed the skills to know that the angle value will be 180-(360/20).
The steps detailed bellow take you from basic identification all the way through to construction of the Regular Decagon shape, and more!
These steps also include "pit stops" to complete fun geometry projects and coloring sheets.
These are nice "breathers" on the learning curve, but they are excellent ways of reinforcing the new knowledge in ways that your kid can get a real life, hands on approach to understanding the basic geometry concepts included.
Okay, so let"s get began ...
Identify - How do we know what we look at is a Regular Decagon?
A Decagon is identified by the number of sides it has.
It has TEN sides.
A Regular Decagon is identified by a combination of the number of sides to the shape, the length of the sides AND the size of its angles.
A Regular Decagon has TEN Sides equal in length ... and TEN Angles equal in size - all angles are 36 degrees at the center (360/10 = 36), with internal angles of 144 degrees <180-(360/10) = 144>
Once your child is comfortable with how to recognize this shape, offer them some shape worksheets to see how they get on with identifying it.
How to calculate the Area and Perimeter of the Regular Decagon.
To find the area of a Regular Decagon we must know two things:
1: The length of one side and,
2: The perpendicular distance from the center of the Decagon to one of its sides. (In "math speak" this perpendicular distance is known as the APOTHEM!) In my diagram I have included 2 (there is a total of 10 in an decagon) and these are the blue lines. You can also see that these Apothem"s are in fact the radius of a circle inscribed in the decagon (constructed in green).
The Area of the decagon is calculated by first finding the area of one of the Isosceles Triangles created by one side of the decagon, and two lines constructed from the center point to each vertex. Then you multiply this answer by 10, as there are ten of these triangles in a decagon.
These construction lines are in Orange in my diagram.
Step 1: Area of Isosceles Triangle
Area of any triangle is half its base multiplied by it perpendicular height. In this case
1/2 a r, where r (the perpendicular height) is the apothem.
Step 2: Multiply by 10
We have 10 of these triangles in the decagon, so to get its area, we must get the area of ALL 10 triangles.
10(1/2 a r)
Tidied up this is equal to 5(ar)
The Perimeter of ANY shape is simply the sum total of all the lengths of the shape - and a Decagon is no different.
The perimeter of a Regular Decagon is
10 times the length of one of its sides..
How to construct a Regular Decagon
To complete this, you will need a ruler, pencil, protractor and a blank piece of paper!
Step 1: Draw a straight line lightly using your ruler and pencil on your paper. - This is what we call a construction line.
Step 2: Indicate on this line, one point - this point will be the first of the ten vertices.
Note:We know that the eight internal angles of any regular pentagon are each 144 degrees.
Step 3: Using your protractor, find the point at 144 degrees to your first line using the point you indicated in the previous step, and draw another construction line.
You now have two lines of your decagon.
Step 4: Using your ruler, measure the length of your decagon side on BOTH of these lines, and mark with a point.
Step 5: Using these new points, repeat Step 3: until you have completed your decagon shape.
Step 6: Your decagon is the shape contained between the ten lines you have constructed.
Step 7: Using a heavier line connect the ten points to finish your construction.
See more: How Do Butterflies Taste With Their Feet, How Do Butterflies Taste Their Food
A quick check to ensure your shape is accurate, is to measure all side lengths with your ruler. If you have done it correctly all sides will measure the same!
Relationship to 3D Shapes
There are no 3d shapes an elementary math student will study involving this shape.