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

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

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

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

a: 2/3=10/15

1/5=3/15

b: 3/4=6/8

5/8=5/8

c: 5/6=15/18

4/9=8/18

\(\dfrac{2}{3}=\dfrac{10}{15};\dfrac{1}{5}=\dfrac{3}{15}\)

\(\dfrac{3}{4}=\dfrac{24}{32};\dfrac{5}{8}=\dfrac{20}{32}\)

\(\dfrac{5}{6}=\dfrac{45}{54};\dfrac{4}{9}=\dfrac{24}{54}\)

a. \(\dfrac{2}{3}=\dfrac{2.5}{3.5}=\dfrac{10}{15};\dfrac{1}{5}=\dfrac{1.3}{5.3}=\dfrac{3}{15}\)

b.\(\dfrac{3}{4}=\dfrac{3.2}{4.2}=\dfrac{6}{8};\dfrac{5}{8}=\dfrac{5}{8}\)

\(\dfrac{5}{6}=\dfrac{5.3}{6.3}=\dfrac{15}{18};\dfrac{4}{9}=\dfrac{4.2}{9.2}=\dfrac{8}{18}\)

\(\frac{1}{4}\)và \(\frac{5}{6}\)

\(\frac{1}{4}\)=\(\frac{1X3}{4X3}\)=\(\frac{3}{12}\)

\(\frac{5}{6}\)=\(\frac{5X2}{6X2}\)=\(\frac{10}{12}\)

1/4 và 5/6 ta quy đồng 4x3 =12 quy đồng 6 x2=12                                                                                                                               ta quy đồng tử số của 1/4và 5/6 ta được phân số 3/12 và 10/12

cảm ơn các bạn 

\(a) {4\over12}={4.5\over12.5}={20\over60}\)

    \({1\over5}={1.12\over5.12}={12\over60}\)

    \({-4\over15}={-4.4\over15.5}={-16\over60}\)

b)\({-7\over10}={-7.3\over10.3}={-21\over20}\)

    \({1\over2}={1.15\over2.15}={15\over30}\)

    \({-4\over15}={-4.2\over15.2}={-8\over30}\)

c)\({-5\over14}={-5.30\over14.30}={-150\over420}\)

    \({3\over20}={3.21\over20.21}={63\over420}\)

    \({-4\over21}={-4.20\over21.20}={-80\over420}\)

d)\({4\over18}={4.5\over18.5}={20\over90}\)

    \({1\over18}={1.18\over5.18}={18\over90}\)

    \({-6\over15}={-6.6\over15.6}={-36\over90}\)

Nói lời giữ lời nha ^_^..........Mệt wá........

a)4/12=1.5/12.5=5/60

  1/5=1.12/5.12=5/60

  -4/15=-4.4/15.4=-16/60

b)-7/10=-7.3/10.3=-21/30

   1/2=1.15/2.15=15/30

   -4/15=-4.2/15.2=-8/30

c)-5/14=-5.30/14/30=150/420

   3/20=3.21/20.21=63/420

   -4/21=-4.20/21.20=-80/420

d)4/18=4.5/18.5=20/90

   1/5=1.18/5.18=18/90

   -6/15=-6.6/15.6=-36/90

a đu, thế mà bảo là lớp 6 hả

Bài 1: 

Các phân số 3/5; 9/20; 17/27 là phân số tối giản

Bài 2: 

a: 1/4=7/28

3/7=12/28

b: 4/5=28/35

17/35=17/35

c: 4/9=28/63

8/21=24/63

d: 5/6=20/24

7/8=21/24

ngu dốt

\(\dfrac{1}{2}và\dfrac{2}{9}\)

\(\dfrac{1}{2}=\dfrac{1.9}{2.9}=\dfrac{9}{18};\dfrac{2}{9}=\dfrac{2.2}{9.2}=\dfrac{4}{18}\)

\(\dfrac{3}{5}và\dfrac{9}{8}\)

\(\dfrac{3}{5}=\dfrac{3.8}{5.8}=\dfrac{24}{40};\dfrac{9}{8}=\dfrac{9.5}{8.5}=\dfrac{45}{40}\)

\(\dfrac{5}{6}và\dfrac{4}{9}\)

\(\dfrac{5}{6}=\dfrac{5.3}{6.3}=\dfrac{15}{18};\dfrac{4}{9}=\dfrac{4.2}{9.2}=\dfrac{8}{18}\)

a)\(\frac{7}{5}=\frac{84}{60}\);\(\frac{3}{15}=\frac{12}{60}\);\(\frac{4}{20}=\frac{12}{60}\)

b)\(\frac{4}{9}=\frac{112}{252}\);\(\frac{2}{7}=\frac{72}{252}\);\(\frac{1}{4}=\frac{63}{252}\)

\(\dfrac{4\times9}{5\times9}=\dfrac{36}{45};\dfrac{7\times5}{9\times5}=\dfrac{25}{45}\\ \dfrac{5\times2}{8\times2}=\dfrac{10}{16};\dfrac{15}{16}\\ \dfrac{1\times40}{6\times40}=\dfrac{40}{240};\dfrac{5\times30}{8\times30}=\dfrac{150}{240};\dfrac{4\times48}{5\times48}=\dfrac{192}{240}\)