Đáp án:
Bài 1:
a, $\frac{5}{7}$ + $\frac{7}{6}$ = $\frac{30}{42}$ + $\frac{49}{42}$ = $\frac{79}{42}$
b, $\frac{-5}{7}$ . $\frac{2}{11}$ + $\frac{-5}{7}$ . $\frac{9}{11}$ + $\frac{5}{7}$
= $\frac{-5}{7}$ . ( $\frac{2}{11}$ + $\frac{9}{11}$ - 1)
= $\frac{-5}{7}$ . 0 = 0
c, $\frac{-4}{5}$ : $\frac{4}{3}$ = $\frac{-4}{5}$ . $\frac{3}{4}$ = $\frac{-3}{5}$
Bài 2:
a, $\frac{3}{4}$ . x = $\frac{1}{2}$ x = $\frac{1}{2}$ : $\frac{3}{4}$ x = $\frac{2}{3}$
b, $\frac{13}{15}$ . x - $\frac{2}{3}$ = $\frac{1}{5}$
$\frac{13}{15}$ . x = $\frac{1}{5}$ + $\frac{2}{3}$
$\frac{13}{15}$ . x = $\frac{13}{15}$
x = 1
Bài 3: A =2/1.3+2/3.5+2/7.5+..........+2/2017.2019
= 1/1 - 1/3 + 1/3 - 1/5 + 1/5 - 1/7 +... + 1/2017 - 1/2019
= 1 - 1/ 2019
= 2018/ 2019