Choose a convincing argument that uses indirect reasoning. show that an equilateral triangle cannot have an obtuse angle. a. assume an angle is obtuse, and therefore has measure greater than 90. since the triangle is equilateral, it is equiangular, and each angle measures 60. therefore, no angles are obtuse. b. construct two triangles with obtuse angles, and measure the sides to prove the triangles are not equilateral. c. construct two equilateral triangles, and measure the resulting angles. you can then generalize that no equilateral triangles will have an obtuse angle.