Two supplementary angles can never be both obtuse. To prove this, we use this example – consider two supplementary angles 60 degrees and 120 degrees. Sum of these two angles is 180 degrees. Here, the angle 120 degrees is an obtuse angle because obtuse angles are greater than 90 degrees. And since the angle 60 degrees is less than 90 degrees, it cannot be obtuse. Thus, the two supplementary angles cannot be both obtuse.