There are several types of triangles in geometry, each with different side lengths and angles in relation to one another, but all triangles have one characteristic in common: They all have three angles that add to 180 degrees. This characteristic allows you to take unknown measurements from a triangle and subtract them from 180 to determine any remaining angles. You can also measure the angles with a protractor and compare.
Align the horizontal edge of the protractor with the base of the triangle.
Place the center point of the protractor on the vertex of the angle.
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Follow the side of the triangle until it reaches the angle measurement mark. Note the measurement.
Repeat for any other angles you want to find.