how do we use the exterior angle theorem?

To use the exterior angle theorem we have to understand what is an exterior angel is
 Exterior Angel -  triangle is equal to the sum of the measures of the two non-adjacent interior angles of the triangle.

The exterior angle theorem is that the exterior angle is the sum the two opposite angle.

for example say <b 60° and <a is 60° the sum of both those angles will equal <d making it 120° this means that <c is 60° (180°-120°= 60°)

for more help or other examples check out these links below:

• http://hotmath.com/hotmath_help/topics/exterior-angle-theorem.html
• http://www.regentsprep.org/Regents/math/geometry/GP5/LExtAng.htm
• http://www.youtube.com/watch?v=YS2K2SpvVIA
• http://classroom.westsidehsfaculty.org/webs/jschroe1/upload/worksheet_triangle_sum_and_exterior_angle.pdf
• http://www.kutasoftware.com/FreeWorksheets/GeoWorksheets/4-The%20Exterior%20Angle%20Theorem.pdf