khó :v

Bài 1 :

Ta có :

\(-\frac{7}{72}>-\frac{9}{72}\)

\(-\frac{9}{72}>-\frac{9}{40}\)

\(\Rightarrow-\frac{7}{72}>-\frac{9}{40}\)

Vậy \(-\frac{7}{72}>\frac{9}{-40}\)

Bài 2 :

218 - ( x + 3 ) = x - 29

  218 - x - 3    = x - 29

  218 - 3 + 29 = x + x

            2x       = 244

              x       = 122

Vậy x = 122

Bài 1:

\(\dfrac{-5}{18}=\dfrac{-20}{72};\dfrac{7}{-24}=\dfrac{-21}{72}.\)

\(\dfrac{-15}{-40}=\dfrac{3}{8}=\dfrac{9}{24};\dfrac{24}{-72}=\dfrac{-1}{3}=\dfrac{-8}{24}.\)

Bài 3:

a) \(\dfrac{2}{3}h=\dfrac{8}{12}h;\dfrac{3}{4}h=\dfrac{9}{12}h.\Rightarrow\dfrac{2}{3}h< \dfrac{3}{4}h.\)

b) \(\dfrac{4}{5}km/h=\dfrac{8}{10}km/h;\dfrac{9}{10}km/h.\Rightarrow\dfrac{4}{5}km/h< \dfrac{9}{10}km/h.\)

a:Cách 1: \(\frac{21}{35}=\frac{21:7}{35:7}=\frac35\)

\(\frac{12}{40}=\frac{12:4}{40:4}=\frac{3}{10}\)

mà \(\frac35>\frac{3}{10}\left(5<10\right)\)

nên \(\frac{21}{35}>\frac{12}{40}\)

Cách 2: \(\frac{21}{35}=\frac{21\times8}{35\times8}=\frac{168}{280}\)

\(\frac{12}{40}=\frac{12\times7}{40\times7}=\frac{84}{280}\)

mà 168>84

nên \(\frac{21}{35}>\frac{12}{40}\)

b: Cách 1: \(\frac{35}{49}=\frac{35:7}{49:7}=\frac57\)

\(\frac{40}{64}=\frac{40:8}{64:8}=\frac58\)

mà \(\frac57>\frac58\left(7<8\right)\)

nên \(\frac{35}{49}>\frac{40}{64}\)

Cách 2: \(\frac{35}{49}=\frac{35\times64}{49\times64}=\frac{2240}{3136}\)

\(\frac{40}{64}=\frac{40\times49}{64\times49}=\frac{1960}{3136}\)

mà 2240>1960

nên \(\frac{35}{49}>\frac{40}{64}\)

c: Cách 1: \(\frac{9}{11}=1-\frac{2}{11}\)

\(\frac{13}{15}=1-\frac{2}{15}\)

Ta có: 11<15

=>\(\frac{2}{11}>\frac{2}{15}\)

=>\(-\frac{2}{11}<-\frac{2}{15}\)

=>\(-\frac{2}{11}+1<-\frac{2}{15}+1\)

=>\(\frac{9}{11}<\frac{13}{15}\)

Cách 2: Ta có: \(\frac{9}{11}=\frac{9\times15}{11\times15}=\frac{135}{165}\)

\(\frac{13}{15}=\frac{13\times11}{15\times11}=\frac{143}{165}\)

mà 135<143

nên \(\frac{9}{11}<\frac{13}{15}\)

d: Cách 1: \(\frac{21}{7}=\frac{21\times5}{7\times5}=\frac{105}{35}\)

mà 105>24

nên \(\frac{21}{7}>\frac{24}{35}\)

cách 2: Ta có: \(\frac{21}{7}=3>1;1>\frac{24}{35}\)

Do đó: \(\frac{21}{7}>\frac{24}{35}\)

e: Cách 1: \(\frac12=\frac{1\times2424}{2\times2424}=\frac{2424}{4848}\)

Cách 2: \(\frac{2424}{4848}=\frac{2424:2424}{4848:2424}=\frac12\)

f: Vì 19>11

nên \(\frac{19}{15}>\frac{11}{15}\)

1: B là số nguyên

=>n-3 thuộc {1;-1;5;-5}

=>n thuộc {4;2;8;-2}

3:

a: -72/90=-4/5
b: 25*11/22*35

\(=\dfrac{25}{35}\cdot\dfrac{11}{22}=\dfrac{5}{7}\cdot\dfrac{1}{2}=\dfrac{5}{14}\)

c: \(\dfrac{6\cdot9-2\cdot17}{63\cdot3-119}=\dfrac{54-34}{189-119}=\dfrac{20}{70}=\dfrac{2}{7}\)

9/7>11/12

2/15<3/12

38/39<39/40