Hãy giải thích vì sao các phân thức sau lại bằng nhau 1) \(\frac{5y}{7} = \frac{20xy}{28x}\) 2) \(\frac{x^2y}{5} = \frac{3xy}{15x}\) 3) \(\frac{1}{(x-y)^2} = \frac{(x-y)}{(x-y)^2}\) 4) \(\frac{(x+5)}{2x(x+5)} = \frac{3}{6x}\) 5) \(\frac{x^2(x+2)}{(x+2)^2} = \frac{x}{x+2}\) 6) \(x\frac{(x^2-1)}{x+1} = x^2 - x\) 7) \(\frac{x^3 + 8}{x^2 - 2x + 4} = x + 2\) 8) \(\frac{3-x}{3+x} = \frac{-x^2-6x+9}{9-x}\) 9) \(\frac{x^2 - x - 2}{x+1} = \frac{x^2 - 3x + 2}{x-1}\)