>>12543868integral from 0 to 1 of g(x) is equal to integral from 0 to 1 of g(1-x) . This is true for any g.
We add these together and get integral from 0 to 1 of (g(x) + g(1-x)) . From your relation that gives integral from 0 to 1 of 1, which gives 1. Dividing by 2 we recover integral from 0 to 1 of g(x) as 1/2.