 some of the interior angles of any triangle is 180 degrees by Euclid's second postulate we can extend this side indefinitely and so we can extend this side indefinitely. By Euclid's fifth postulate there is a line containing this point that is parallel to this side of the triangle. Now let's call this a let's call this angle b and let's call this angle c we can see that this angle is congruent to this angle let's call it b prime we also see that this angle is congruent to this angle let's call it a prime and this angle is congruent to this angle let's call it c prime since the sum of a plus b of a b and c is 180 degrees the sum of a prime plus b prime plus c prime must also be 180 degrees thank you