39. Сократите: а) $$rac{4\cdot 5}{15\cdot 6}$$; б) $$rac{12\cdot 14}{49\cdot 15}$$; в) $$rac{3\cdot 15 - 3\cdot 7}{27}$$

а) $$rac{4\cdot 5}{15\cdot 6} = \frac{4\cdot 5}{3\cdot 5\cdot 2\cdot 3} = \frac{4}{3\cdot 2\cdot 3} = \frac{2\cdot 2}{3\cdot 2\cdot 3} = \frac{2}{9}$$ б) $$\frac{12\cdot 14}{49\cdot 15} = \frac{3\cdot 4\cdot 2\cdot 7}{7\cdot 7\cdot 3\cdot 5} = \frac{4\cdot 2}{7\cdot 5} = \frac{8}{35}$$ в) $$\frac{3\cdot 15 - 3\cdot 7}{27} = \frac{3(15-7)}{27} = \frac{3\cdot 8}{27} = \frac{3\cdot 8}{3\cdot 9} = \frac{8}{9}$$