Proving the Interior Angle Sum Theorem

Proving the Interior Angle Sum TheoremAdapted from Walch Education

1.9.1: Proving the Interior Angle Sum Theorem2Triangles Classified by Angle Measure

Acute triangleObtuse triangleRight triangleAll anglesare less than 90.One angleis greater than 90.One anglemeasures 90.

1.9.1: Proving the Interior Angle Sum Theorem3Triangles Classified by Side Lengths

Scalene triangleIsosceles triangleEquilateral triangleNo congruent sides

At least two congruent sides

Three congruent sides

1.9.1: Proving the Interior Angle Sum Theorem4Triangle Sum Theorem

TheoremTriangle Sum Theorem The sum of the angle measures of a triangle is 180.

mA + mB + mC = 180

The Triangle Sum Theorem can be proven using the Parallel Postulate.The Parallel Postulate states that if a line can be created through a point not on a given line, then that line will be parallel to the given line.This postulate allows us to create a line parallel to one side of a triangle to prove angle relationships.

1.9.1: Proving the Interior Angle Sum Theorem5Triangle Sum Theorem1.9.1: Proving the Interior Angle Sum Theorem6Parallel Postulate

PostulateParallel Postulate Given a line and a point not on it, there exists one and only one straight line that passes through that point and never intersects the first line.

Notice that C and D are supplementary; that is, together they create a line and sum to 180.

1.9.1: Proving the Interior Angle Sum Theorem7Interior Angles, Exterior Angles, and Remote Interior Angles

Interior angles: A, B, and C Exterior angle: DRemote interior angles of D: A and B

1.9.1: Proving the Interior Angle Sum Theorem8Exterior Angle Theorem

TheoremExterior Angle Theorem The measure of an exterior angle of a triangle is equal to the sum of the measures of its remote interior angles.

mD = mA + mB

1.9.1: Proving the Interior Angle Sum Theorem9Exterior Angle Inequality Theorem

TheoremExterior Angle Inequality Theorem If an angle is an exterior angle of a triangle, then its measure is greater than the measure of either of its corresponding remote interior angles.

mD > mA mD > mB

1.9.1: Proving the Interior Angle Sum Theorem10More Theorems

TheoremIf one angle of a triangle has a greater measure than another angle, then the side opposite the greater angle is longer than the side opposite the lesser angle.

mA < mB < mC a < b < c

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