Tính nhanh các biểu thức sau

Bài 1: Tính nhanh
$$\begin{gathered} \mathrm{A} = \frac{5}{13} + \frac{-5}{7} + \frac{-20}{41} + \frac{8}{13} + \frac{-21}{41} \\ \mathrm{B} = \frac{-2}{5} + \frac{3}{4 } + \frac{6}{7} + \frac{3}{4} +\frac{2}{5} \\ \mathrm{C} = \frac{6}{7} + \frac{1}{7}.\frac{2}{7} + \frac{1}{7}.\frac{5}{7} \\ \mathrm{D} = \frac{5}{7}.\frac{5}{11} + \frac{5}{7}.\frac{2}{11} - \frac{5}{7}. \frac{14}{11} \\ \mathrm{GIÚP} \mathrm{MK} \mathrm{VS} \mathrm{MK} \mathrm{ĐAG} \mathrm{CẦN} \mathrm{GẤP} \end{gathered}$$