Basic Theorems of Triangles Sophomore Geometry By: Shannon Manzella

Interior Angles Theorem In a Triangle, the Interior Angles will always add to be 180° – The Interior Angles are the inside angles between adjacent sides of a rectilinear figure.

Example A

Base Angles Theorem If the sides of a triangle are congruent ( like in an Isosceles triangle) then the angles opposite these sides are congruent. – Two angles are congruent if they have the value and size. Leg A Leg B Base Angle B Base Angle A

Example B

Example C

The Midpoint Theorem The segment joining the midpoints of two sides of a triangle is parallel to the third side and half as long as the third side. – The Midpoint of a line segment is the point that is halfway between the endpoints of the line segment.

Example D

Example E

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