Bài 1. Thực hiện các phép tính a) \( \frac{3x+1}{x+y} - \frac{2x-3}{x+y} \) b) \( \frac{x+3}{x^2-1} + \frac{1}{x^2+x} \) c) \( \frac{x}{x-3} - \frac{9}{x^2-3x} \) d) \( \frac{5x+10}{4x-8} - \frac{2x}{x+2} \) e) \( \frac{5x-15}{4x+4} - \frac{9}{x^2+2x+1} \) f) \( \left( \frac{1}{x-1} - \frac{x^2+x+1}{1-x^3} \right) \cdot \frac{2x+1}{x^2+2x+1} \)