The symmetry axis of quadratic function f (x + a) = f (b-X) is x = (a + b) / 2. Please give a detailed explanation,

The symmetry axis of quadratic function f (x + a) = f (b-X) is x = (a + b) / 2. Please give a detailed explanation,

Just prove that f (x) = f (a + b-X)
F (x) = f [(x-a) + a] = f [b - (x-a)] = f (a + b-X)