These free geometry worksheets will introduce you to the Triangle Sum Theorem, as you find the measurements of the interior angles of a triangle.  The interior, or inside, angles of a triangle always add up to 180 degrees.

In addition to your geometry skills, you’ll be able to polish your algebra skills as you set up and solve equations on some of the tougher triangles.  That’s one great way to check your answer on these exercises, if the sum of your angles is 180, then you got ’em all correct.

Sum of the Interior Angles of a Triangle Worksheet 1 – This angle worksheet features 12 different triangles.  The measures of two angles are given and the third angle measurement is unknown.

Sum of the Interior Angles of a Triangle Worksheet 1 RTF
Sum of the Interior Angles of a Triangle Worksheet 1 PDF
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Sum of the Interior Angles of a Triangle Worksheet 2 – This angle worksheet features 12 different triangles.  The measure of one angle is given, the other two angles are represented by algebraic expressions like 5x and x + 7.  You can solve these problems in a variety of ways, my favorite is to set up and solve an equation that’s equal to 180, like 5x + x + 7 + 75 = 180.

Sum of the Interior Angles of a Triangle Worksheet 2 RTF
Sum of the Interior Angles of a Triangle Worksheet 2 PDF
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Sum of the Interior Angles of a Triangle Worksheet 3 – This angle worksheet features 12 different triangles.  The measure of each angle is represented by an algebraic expression.  That’s right, you’re not give the measure of any of the three angles in the triangle.  If you paid attention on the previous worksheet, this one should be a piece of cake!

Sum of the Interior Angles of a Triangle Worksheet 3 RTF
Sum of the Interior Angles of a Triangle Worksheet 3 PDF
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