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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.

Angles of a triangle
Free geometry lesson plan from HomeschoolMath.net

In the previous study you probably made a guess that the sum of the angles in a triangle is 180.  That is true.  Here is a proof for it.  Look at the picture.   We draw a line parallel to AB that passes through C.   The three angles of the triangle can also be found above that line as the picture indicates, and there they three form together a 180 angle.

Example problem types

1.  Calculate the angle marked with the question mark.  Note: do not measure since the pictures are not exact.  What principles should you use?

[Pictures available in the ebook]

2.  Draw a triangle whose all three angles have the same measure.  What is that measure?

3.  Try to draw a triangle that has two right angles.  Can you do it?  Why?

6.  Draw a triangle whose one angle is 35 and another angle is 85.  Can you draw the triangle without calculating the measure of the third angle?

Calculate (do not measure) the angle marked with ?.

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