Why can't a quadrilateral have four obtuse angles?

Answer 1

The sum of the 4 internal angles of a quadrilateral must sum to 360° (go look up the proof of this elsewhere).

For the sake of argument, let's call the 4 internal angles of a quadrilateral A, B, C, and D.

If they were all obtuse, then (by definition):

180° > A > 90°

180° > B > 90°

180° > C > 90°

180° > D > 90°

If you add them all up, you find that for the sum of the angles A+B+C+D

720° > A + B + C + D > 360°

but, A+B+C+D = 360°,

Since the assumption produces a result that contradicts this, the assumption cannot be true.

Q.E.D.