D

Gọi phân số cần tìm là \(\frac{7}{a}\)

Theo bài ra ta có:

\(\frac{-9}{10}<\frac{7}{a}<\frac{-9}{11}\)

=>\(\frac{9}{-10}<\frac{7}{a}<\frac{9}{-11}\)

=>\(\frac{63}{-70}<\frac{63}{a}<\frac{63}{-77}\)

=>\(-70>a>-77\)

=>\(a=-71,-72,-73,-74,-75,-76\)

Vậy những phân số cần tìm là \(\frac{7}{-71},\frac{7}{-72},\frac{7}{-73},\frac{7}{-74},\frac{7}{-75},\frac{7}{-76}\)

Câu 2:

Goi phân số cần tìm là: a/b; (a; b ∈ z; b ≠ 0)

Theo bài ra ta có: -9/10 < 7/b < -9/11

-63/70 < 63/9b < -63/77

- 70 > 9b > -77

-70/9 > b > -77/9

- 7\(\frac79\) > b > - 8\(\frac59\)

vì b nguyên nên: b = - 8

Vậy phân số cần tìm là: -7/8

1: Rút gọn

a) \(-\dfrac{33}{51}=\dfrac{-33:3}{51:3}=\dfrac{-11}{17}\)

b) \(\dfrac{156}{-168}=\dfrac{-156}{168}=\dfrac{-156:12}{168:12}=\dfrac{-13}{14}\)

c) \(\dfrac{-75}{-100}=\dfrac{75}{100}=\dfrac{75:25}{100:25}=\dfrac{3}{4}\)

2: Quy đồng mẫu:

a) \(-\dfrac{3}{8}\) và \(\dfrac{5}{4}\)

MSC: 8

\(-\dfrac{3}{8}=\dfrac{-3}{8}\)

\(\dfrac{5}{4}=\dfrac{5\cdot2}{4\cdot2}=\dfrac{10}{8}\)

b) \(\dfrac{-7}{6}\) ; \(\dfrac{5}{12}\) và \(-\dfrac{5}{6}\)

MSC: 12

\(-\dfrac{7}{6}=\dfrac{-7\cdot2}{6\cdot2}=\dfrac{-14}{12}\)

\(\dfrac{5}{12}=\dfrac{5}{12}\)

\(\dfrac{-5}{6}=\dfrac{-5\cdot2}{6\cdot2}=\dfrac{-10}{12}\)

\(\dfrac{7}{-18}< \dfrac{-7}{24}\)

\(\dfrac{7}{-18}\) và \(\dfrac{-7}{24}\) \(=\dfrac{-7}{18};\dfrac{-7}{24}=\dfrac{-28}{72};\dfrac{-21}{72}=\dfrac{-28}{72}< \dfrac{-21}{72}\)

Vậy \(\dfrac{7}{-18}< \dfrac{-7}{24}\)

\(\dfrac{-3}{7}\)

\(\dfrac{5}{22}\)

\(\dfrac{12}{7}\)

\(\dfrac{28}{17}\)

\(\dfrac{11}{12}\)

\(\dfrac{-3}{7};\dfrac{5}{22};\dfrac{12}{7};\dfrac{28}{17};\dfrac{11}{12}\)