F (x) and G (x) are functions defined on R, the equation x-f (g (x)) = 0, G (f (x) can not be A X^2+X-1\5 Bx^2+x+1\5 Cx^2-1\5 DX^2+1\5 Let f (x) = x, then G (f (x) = g (x) = f (g (x)) can be obtained I want to ask 1 why we can set f (x) = x, is it because of the equation, I personally think f (x) should not be equal to x, but it is If f (g (x)) = x can be regarded as f (x) = x, and the rule of F is not x, how should we look at it

F (x) and G (x) are functions defined on R, the equation x-f (g (x)) = 0, G (f (x) can not be A X^2+X-1\5 Bx^2+x+1\5 Cx^2-1\5 DX^2+1\5 Let f (x) = x, then G (f (x) = g (x) = f (g (x)) can be obtained I want to ask 1 why we can set f (x) = x, is it because of the equation, I personally think f (x) should not be equal to x, but it is If f (g (x)) = x can be regarded as f (x) = x, and the rule of F is not x, how should we look at it

1. F (x) =? I don't know, do I? Then I can guess... I guess f (x) = x is OK, OK, try it first, won't it?
2. The relationship between functions is not clear... G (x) is defined on R, which can only show that the value range a of G (x) is a subset of R. this can be understood. If you don't understand it, baidu function, or the relationship between value range and definition under Baidu, which clearly shows that it is only a subset
3. F (g (x)) = x can't be regarded as f (x) = x, which has been explained in (1), but it is conjectured that f (x) = x, and there is no practical connection between them