Cho 2 biểu thức: A = x - 3/ x^2 - x + 1                                                                                                B = (3x + 6/ x^2 - 9 - 2/ x - 3) : 1/ x +3                                                                                                        a) Chứng minh: B = x/ x - 3                                                                                                                                                                                                  b)      Tính P = A.B