- Đặt tính: Viết các số cùng hàng thẳng cột với nhau.

- Tính: Lần lượt cộng hoặc trừ các số cùng hàng theo thứ tự từ phải sang trái.

a: 12/21=4/7=8/14

4/56=1/14

b: 12/15=4/5=24/30

6/36=1/6=5/30

15/50=3/10=9/30

c: 63/77=9/11=81/99

60/108=5/9=55/99

-18/27=-2/3=-66/99

d: -26/-156=1/6=7/42

49/-14=-7/2=-147/42

32/112=2/7=12/42

\(\dfrac{90}{120}=\dfrac{30.3}{30.4}=\dfrac{3}{4}\)

\(\dfrac{15}{25}=\dfrac{5.3}{5.5}=\dfrac{3}{5}\)

\(\dfrac{18}{45}=\dfrac{9.2}{9.5}=\dfrac{2}{5}\)

\(\dfrac{36}{64}=\dfrac{4.9}{4.16}=\dfrac{9}{16}\)

\(\dfrac{32}{72}=\dfrac{8.4}{8.9}=\dfrac{4}{9}\)

\(\dfrac{84}{49}=\dfrac{7.12}{7.7}=\dfrac{12}{7}\)

\(\dfrac{90}{120}=\dfrac{3}{4};\dfrac{15}{25}=\dfrac{3}{5};\dfrac{18}{45}=\dfrac{2}{5};\dfrac{36}{64}=\dfrac{9}{16};\dfrac{32}{72}=\dfrac{4}{9};\dfrac{84}{49}=\dfrac{12}{7}\)

\(\dfrac{3}{4},\dfrac{3}{5},\dfrac{2}{5},\dfrac{9}{16},\dfrac{4}{9},\dfrac{12}{7}\)

\(\dfrac{5}{15}+\dfrac{48}{16}=\dfrac{1}{3}+3=\dfrac{1}{3}+\dfrac{9}{3}=\dfrac{10}{3}\)

\(\dfrac{6}{9}:\dfrac{12}{36}=\dfrac{2}{3}:\dfrac{1}{3}=\dfrac{2}{3}\times\dfrac{3}{1}=\dfrac{6}{3}=2\)

\(\dfrac{4}{12}\times\dfrac{56}{14}=\dfrac{1}{3}\times4=\dfrac{4}{3}\)

Lời giải:
$\frac{20}{12}+\frac{36}{18}-\frac{21}{28}=\frac{5}{3}+2-\frac{3}{4}$

$=\frac{20}{12}+\frac{24}{12}-\frac{9}{12}=\frac{20+24-9}{12}=\frac{35}{12}$

\(=\dfrac{5}{3}+2-\dfrac{3}{4}=\dfrac{5.4+2.12-3.3}{12}=\dfrac{20+24-9}{12}=\dfrac{35}{12}\)

a: =2/3-1/3=1/3

b: =3/5-1/7=21/35-5/35=16/35

c: =2/3-1/2=4/6-3/6=1/6

tự làm , ngu

b: 12/20+6/18=3/5+1/3=9/15+5/15=14/15

c: 8/12+20/30=2/3+2/3=4/3

d: 10/12+12/36=5/6+2/6=7/6

\(\dfrac{6}{36}+\dfrac{5}{6}=\dfrac{1}{6}+\dfrac{5}{6}=\dfrac{6}{6}=1\\ \dfrac{1}{4}+\dfrac{5}{30}=\dfrac{1}{4}+\dfrac{1}{6}=\dfrac{6}{24}+\dfrac{4}{24}=\dfrac{10}{24}=\dfrac{5}{12}\\ \dfrac{5}{56}+\dfrac{3}{21}=\dfrac{5}{56}+\dfrac{1}{7}=\dfrac{5}{56}+\dfrac{8}{56}=\dfrac{13}{56}\\\dfrac{12}{18}+\dfrac{12}{42}=\dfrac{2}{3}+\dfrac{2}{7}=\dfrac{14}{21}+\dfrac{6}{21}=\dfrac{20}{21} \)

6/36 + 5/6=1/6+5/6=6/6=1

1/4 + 5/30=1/4+1/6=6/24+4/24=10/24=5/12

5/56 + 3/21=5/56=1/7=5/56+8/56=13/56

12/18 + 12/42=2/3+2/7=14/21+6/21=20/21

\(a,\dfrac{6}{36}+\dfrac{5}{6}=\dfrac{1}{6}+\dfrac{5}{6}=\dfrac{6}{6}=1\\ b,\dfrac{1}{4}+\dfrac{5}{30}=\dfrac{1}{4}+\dfrac{1}{6}=\dfrac{6}{24}+\dfrac{4}{24}=\dfrac{5}{12}\\ c,\dfrac{5}{56}+\dfrac{3}{21}=\dfrac{5}{56}+\dfrac{8}{56}=\dfrac{13}{56}\\ d,\dfrac{12}{18}+\dfrac{12}{42}=\dfrac{14}{21}+\dfrac{6}{21}=\dfrac{20}{21}\)

a, 15/25 - 3/21 = 3/5 - 1/7 = 21/35 - 5/35 = 18/35

b, 24/36 - 6/12 = 2/3 - 1/2 = 4/6 - 3/6 = 1/6

a)15/25-3/21=3/5-1/7=16/35                b)24/36-6/12=2/3-1/2=1/6

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