Question

Write an indirect proof proving only one angle of an obtuse triangle is greater than 90°.

Answer 1

All angles must sum to 180
Angle 1+Angle 2+Angle 3=180

By definition, Angle 3>90 so plug in 90 (lowest possible bound for angle 3) into equation:
Angle 1+Angle 2+ 90=180

Subtract 90 from both sides
Angle 1+Angle 2=90

Since Angle 1 and Angle 2 are greater than zero, neither one can be greater than 90 because their sum has to be, at the most, 90. Hoped I helped!