Thực hiên phép tính a) (1+2+3+...+90) . ( 12.34-6.68) : $ ( \frac{1}{3} + \frac{1}{4} + \frac{1}{5} + \frac{1}{6} )$b) $ \frac{\frac{3}{7} - \frac{3}{11} + \frac{3}{13}}{\frac{5}{7} - \frac{5}{11} + \frac{5}{13} } + \frac{\frac{1}{2} - \frac{1}{3} + \frac{1}{4}}{\frac{5}{4} - \frac{5}{6} + \frac{5}{8}}$