If f (x) = ax square plus bxjia 3A plus B is even function, and the domain of definition is (a-1,2a), then how much is a equal to? How much is B equal to?

If f (x) = ax square plus bxjia 3A plus B is even function, and the domain of definition is (a-1,2a), then how much is a equal to? How much is B equal to?

If f (x) is an even function, then f (x) = f (- x), then 2bx = 0, because it holds in the domain of definition, then B = 0
If f (x) is an even function, then the domain of definition is (a-1,2a) should be symmetric about the origin, then A-1 + 2A = 0, that is, a = 1 / 3
So f (x) = 1 / 3x ^ 2 + 1