ANGLES OF A TRIANGLE

Section 4.2

Angles of a Triangle

Interior angles Original three angles of a triangle

Exterior angles Angles that are adjacent to interior angles

Theorem 4.2

Triangle Sum Theorem The sum of the measures of the interior angles of a

triangle are 180°

m<1=m<2=m<3=

Theorem 4.3

Third Angles TheoremIf two angles of one triangle are congruent to two angles of a second triangle, then the third angles are also congruent

Theorem 4.4The acute angles of a right

triangle are complementary.

More Theorems

Theorem 4.5 Exterior Angle Theorem:The measure of an exterior angle of a

triangle is equal to the sum of the measures of the two remote interior angles.

m<1= m<B + m<A

Thms. Cont.

Theorem 4.6: Exterior Angle Inequality The measure of an exterior angle of a triangle is

greater than the measure of either of the two remote interior angles.

m<1 > m<B or m<A

Finding Angle Measures