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The ideas in this geometry lesson are taken from the Geometry ebook that I sell at MathMammoth.com. This lesson plan does not contain all the problems the Geometry ebook does.

Triangles
Free geometry lesson plan from HomeschoolMath.net

Triangles are closed figures with three sides. They can be classified according to what kind of angle they have.    right angles (exactly 90)   obtuse angles (more than 90, less than 180) acute angles. (less than 90)   Right triangles have exactly one right angle.    Obtuse triangles have exactly one obtuse angle. Acute triangles have three acute angles. In other words, ALL the angles are acute.

Example problem types

1.  Draw a right angle.  Then make a right triangle out of it by drawing in the third side.  Draw two more right triangles with different sizes or positions.  Then measure all the angles in all three triangles.  Are those angles acute, right, or obtuse?  What is the sum of the angles within each triangle?

The other two angles in a right triangle are _____________________.

2.  Draw an obtuse angle.  Then make an obtuse triangle out of it by drawing in the third side.  Draw two more obtuse triangles with different sizes or positions.  Then measure all the angles in all three triangles.  Are those angles acute, right, or obtuse?  What is the sum of the angles within each triangle?

The other two angles in an obtuse triangle are _____________________.

3.  Based on your calculations in exercises 1 and 2, make a guess for the sum of the angles in a triangle.  

5.  Draw a triangle whose one angle is 55 and other angle is 35.  Measure the third angle.  Its measure is ______ degrees.  What kind of triangle did you get (acute, right, obtuse)?

8.  Measure some (or all) angles and some (or all) sides of this triangle and then draw it on your own paper.   In other words, copy the triangle.  Your triangle should match exactly with this triangle if they were placed on top of each other.

Mark the angles and sides first Then measure, and write down your measurements here:

 

9.  These two pictures illustrate how it is ENOUGH to measure just two angles and the side between them to copy a triangle.  In other words, you don't have to know all the angles and sides to copy the triangle. 

Measure the two angles and the one side between the angles from the triangle, and then use that information to copy the triangle to your notebook.  When drawing the triangle, start out by drawing the one side you measured.

[Pictures available in the ebook]

 

12.  a)  Draw a triangle that has a 25 angle, a 115 angle, and a side of 3 inches between those angles.
       b)  Draw a triangle that has a 67 angle, a 75 angle, and a side of 2 inches between those angles.

13.  Draw any parallelogram that is not a rectangle.  Then draw another copy of it in your notebook so that you now have two identical parallelograms side by side.

Now draw a diagonal into it, that is a line from one corner to the opposite corner.  That forms two triangles.  What kind of triangles are they?  (acute, right, obtuse)  Draw the other diagonal into the second parallelogram.  What kind of triangles did you get now?  (acute, right, obtuse)

[Pictures available in the ebook]

Then draw a rectangle, and again divided it into two triangles by drawing the diagonal.  What kind of triangles did you get now?

New terms to remember: a acute triangle a right triangle an obtuse triangle

Next geometry lesson

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