Tính các tổng sau một cách hợp lí:

$\begin{array}{l}a)\frac{1}{2}+\frac{-2}{4}+\frac{4}{8};\\ \\ b)\frac{-3}{9}+\frac{16}{40}+\frac{-8}{12}+\frac{2}{10};\\ \\ c)\frac{-1}{8}+\frac{6}{7}+\frac{2}{14}+\frac{-7}{8}+\frac{7}{9};\\ \\ d)\frac{1}{2}+\frac{-1}{5}+\frac{-5}{7}+\frac{1}{6}+\frac{-3}{35}+\frac{1}{3}\\ \\ e)\frac{-1}{2}+\frac{1}{3}+\frac{-1}{4}+\frac{-2}{8}+\frac{4}{18}+\frac{4}{9}\end{array}$