Indirect Proofs... UGH.

Sorry, rarinstraw.....I couldn't get this to display properly......I hope you can make it out, though

I think that you can select "edit" on my post and see the original format......it will not let you change it......!!!!

I'll try, rarinstraw

(1) Let ABC   be a triangle                                                 Given

(2) Let angle A  be obtuse                                                 Given

(3) Assume that angle B is a right angle                            Indirect proof assumption

(4) Sum of angles A, B  > two right angles                        Definition of obtuse angle, right angle

(5) Sum of any two angles in a triangle < two right angles     Euclid, Book I, Prop. 17  

(6)  Cotradiction between (4) and (5)                                The sum of two angles cannot be > than                                                                                            two right angles and  < than two right

                                                                                           angles at the same time

(7) Angle B is less than a right angle                                 Negation of (3)

 Cphill, Thank you! I could read it perfectly, even if it didn't format correctly.