Parallel Lines and the Triangle Angle Sum Theorem Geometry 3-3

Exterior Angle – The angle formed outside the triangle, and along a side as shown Triangle Term

Exterior Angle – Exterior Angle Triangle Term

Exterior Angle – Six different exterior angles can be formed for a triangle Triangle Term

Investigation Supplies Scissors Printer Paper Draw two different shaped triangles with a ruler (be creative) Cut them out with a scissors Investigation

With ONE of the triangles, tear off the corners of the triangles Arrange the angles so they are adjacent What do you observe? Investigation

With the remaining triangle, tear off two corners of the triangles Arrange the torn corners in the place of an exterior angle to the remaining angle What do you observe? Investigation

What we have learned Triangle Sum Theorem The sum of the measure of the interior angles of a triangle is 180° What we have learned

Alternate Interior Angles Triangle Sum Theorem How can this be proven with parallel lines? y° Z° Alternate Interior Angles What we have learned

What we have learned Exterior Angle Theorem The measure of an exterior angle of a triangle is equal to the measure of the two non-adjacent interior angles What we have learned

Practice Problems

m1=75° m6=40° m2=55° m7=75° m3=55° m8=65° m4=40° m9=115° m5=140°

X=23 Exterior Angle = 100°

mt = 62° mv = 118° mw = 37°

Pages 135 – 138 6 – 11, 16 – 22 even, 27, 28, 33, 36, 49 Homework