a, A=$\frac{4}{7}$ +$\frac{3}{4}$ +$\frac{2}{7}$ +$\frac{5}{4}$ +$\frac{1}{7}$
=($\frac{4}{7}$+$\frac{2}{7}$+$\frac{1}{7}$ )+($\frac{3}{4}$ + $\frac{5}{4}$)
= 1+2=3
Vậy A=3
b, $\frac{1}{5}$ +$\frac{-1}{6}$ +$\frac{1}{7}$+ $\frac{-1}{8}$ +$\frac{1}{9}$ +$\frac{1}{8}$ +$\frac{-1}{7}$ +$\frac{1}{6}$ +$\frac{-1}{5}$
=( $\frac{1}{5}$ +$\frac{-1}{5}$) +($\frac{1}{6}$+ $\frac{-1}{6}$) +($\frac{1}{7}$ +$\frac{-1}{7}$) +($\frac{-1}{8}$ +$\frac{1}{8}$) +$\frac{1}{9}$
=$\frac{1}{9}$
c, $\frac{1}{6}$+ $\frac{1}{12}$+ $\frac{1}{20}$+ $\frac{1}{30}$+$\frac{1}{42}$ +$\frac{1}{56}$
= $\frac{1}{2.3}$+ $\frac{1}{3.4}$+ $\frac{1}{4.5}$+ $\frac{1}{5.6}$+$\frac{1}{6.7}$ +$\frac{1}{7.8}$
= $\frac{1}{2}$- $\frac{1}{8}$
= $\frac{3}{8}$
d, D=($\frac{1}{2}$-1).( $\frac{1}{3}$-1).( $\frac{1}{4}$-1)....( $\frac{1}{1963}$ -1)
=$\frac{-1}{2}$. $\frac{-2}{3}$. $\frac{-3}{4}$ .$\frac{-1962}{1963}$
=$\frac{-1}{1963}$
Vậy D=$\frac{-1}{1963}$
Chúc bạn học tốt! Cho mình 5 sao, cảm ơn và câu trả lời hay nhất nhé