Let's find an equation first, and let the order of three internal angles from big to small be: A, B and C.

Then it can be concluded that a-b = b-c; Then a+c = 2b can be obtained; And because A+B+C= 180 degrees, we can get 3B= 180 degrees and B=60 degrees. So A+C= 120 degrees, where a is an obtuse angle and must be greater than 90 degrees, so the value range of a is 90.

Do you see it?