Tính tổng sau: A = x^4 - (x - 1)^2/(x^2 + 1)^2 - x^2 + x^2 - (x^2 - 1)^2/x^2(x + 1)^2 - 1 + x^2 - (x - 1)^2 - 1/x^4 - (x + 1)^2