`@` `\text {Ans}`

`\downarrow`

`a)`

`25,7 + 9,48 + 14,3`

`= (25,7 + 14,3) + 9,48`

`=40 + 9,48`

`= 49,48`

`b)`

`8,24 + 3,69 + 2,31`

`= 8,24 + (3,69 + 2,31)`

`= 8,24 + 6`

`= 14,24`

`c)`

`8,65 + 7,6 + 1,35 + 0,4`

`= (8,65 + 1,35) + (7,6 + 0,4)`

`= 10 + 8`

`= 18`

`d)`

`5,92 + 0,44 + 5,56 + 4,08`

`= (5,92 + 4,08) + (0,44 + 5,56)`

`= 10 + 6`

`= 16`

`e)`

`7,5 + 6,5 + 5,5 + 4,5 + 3,5 + 2,5`

`= (7,5 + 2,5) + (6,5 + 3,5) + (5,5 + 4,5)`

`= 10 + 10 + 10`

`= 30`

`7,`

`a)`

Ta có:

\(42,54 > 42,45; 87,65 > 87,56\)

\(\Rightarrow\) \(42,54+87,65>42,56+87,56\) 

`b)`

\(96,38+74,85 = 171,73 \\ 74,38 + 96,85 = 171,23\)

\(\Rightarrow\) \(96,38 + 74,85 = 96,85 + 74,38\)

`c)`

\(8,8 + 6,6 + 4,4 = 8,8 + 11 = 19,8 \\ 9,9 + 5,5 + 7,7 = 23,1\)

`19,8 < 23,1`

\(\Rightarrow\) `8,8 + 6,6 + 4,4 < 9,9 + 5,5 + 7,7`

`@` `\text {Ans}`

`\downarrow`

`1)`

\(A=\dfrac{5}{7}+\dfrac{3}{4}+\dfrac{1}{5}+\dfrac{2}{7}+\dfrac{1}{4}\)

`=`\(\left(\dfrac{5}{7}+\dfrac{2}{7}\right)+\left(\dfrac{3}{4}+\dfrac{1}{4}\right)+\dfrac{1}{5}\)

`=`\(1+1+\dfrac{1}{5}=\dfrac{11}{5}\)

Vậy, `A=`\(\dfrac{11}{5}\)

`2)`

\(B=4\dfrac{4}{9}\div\dfrac{8}{3}+\dfrac{2}{3}\div2\dfrac{2}{3}\)

`=`\(\dfrac{40}{9}\times\dfrac{3}{8}+\dfrac{2}{3}\times\dfrac{3}{8}\)

`=`\(\dfrac{3}{8}\times\left(\dfrac{40}{9}+\dfrac{2}{3}\right)\)

`=`\(\dfrac{3}{8}\times\dfrac{46}{9}=\dfrac{23}{12}\)

Vậy, `B =`\(\dfrac{23}{12}\)

`3)`

\(C=\dfrac{2}{15}\times\dfrac{13}{27}+\dfrac{2}{15}\times\dfrac{14}{27}\)

`=`\(\dfrac{2}{15}\times\left(\dfrac{13}{27}+\dfrac{14}{27}\right)\)

`=`\(\dfrac{2}{15}\times1=\dfrac{2}{15}\)

Vậy, `C=`\(\dfrac{2}{15}\)

`4)`

\(D=40,35+112,37-18,35+17,63\)

`=`\(\left(40,35-18,35\right)+\left(112,37+17,63\right)\)

`= 22+130`

`= 152`

Vậy, `D = 152.`

`@` `\text {Ans}`

`\downarrow`

`3,`

`a)`

\(\dfrac{5}{2}\times\dfrac{1}{3}+\dfrac{1}{4}=\dfrac{5}{6}+\dfrac{1}{4}=\dfrac{13}{12}\)

`b)`

\(\dfrac{6}{5}+\dfrac{3}{5}\div2=\dfrac{6}{5}+\dfrac{3}{10}=\dfrac{3}{2}\)

`c)`

\(\left(\dfrac{6}{15}+\dfrac{3}{15}\right)\times\dfrac{4}{9}=\dfrac{9}{15}\times\dfrac{4}{9}=\dfrac{4}{15}\)

`d)`

\(\dfrac{3}{7}\times\left(\dfrac{9}{11}-\dfrac{2}{11}\right)=\dfrac{3}{7}\times\dfrac{7}{11}=\dfrac{3}{11}\)

`4,`

`a)`

\(\dfrac{3}{7}+\dfrac{4}{9}+\dfrac{4}{7}+\dfrac{5}{9}=\left(\dfrac{3}{7}+\dfrac{4}{7}\right)+\left(\dfrac{4}{9}+\dfrac{5}{9}\right)=\dfrac{7}{7}+\dfrac{9}{9}=1+1=2\)

`b)`

\(\dfrac{1}{5}+\dfrac{4}{11}+\dfrac{4}{5}+\dfrac{7}{11}=\left(\dfrac{1}{5}+\dfrac{4}{5}\right)+\left(\dfrac{4}{11}+\dfrac{7}{11}\right)=\dfrac{5}{5}+\dfrac{11}{11}=1+1=2\)

`c)`

\(\dfrac{1}{15}+\dfrac{4}{15}+\dfrac{5}{18}+\dfrac{7}{18}=\left(\dfrac{1}{15}+\dfrac{4}{15}\right)+\left(\dfrac{5}{18}+\dfrac{7}{18}\right)=\dfrac{1}{3}+\dfrac{2}{3}=1\)

`d)`

\(\dfrac{1}{8}+\dfrac{1}{12}+\dfrac{3}{8}+\dfrac{5}{12}=\left(\dfrac{1}{12}+\dfrac{5}{12}\right)+\left(\dfrac{1}{8}+\dfrac{3}{8}\right)=\dfrac{1}{3}+\dfrac{1}{2}=\dfrac{5}{6}\)

`@` `\text {Ans}`

`\downarrow`

`1,`

`a)`

\(\dfrac{7}{19}+\dfrac{12}{19}=\dfrac{7+12}{19}=\dfrac{19}{19}=1\)

`b)`

\(\dfrac{5}{13}+\dfrac{4}{13}=\dfrac{5+4}{13}=\dfrac{9}{13}\)

`c)`

\(\dfrac{24}{11}-\dfrac{13}{11}=\dfrac{24-13}{11}=\dfrac{11}{11}=1\)

`d)`

\(\dfrac{35}{8}-\dfrac{19}{8}=\dfrac{35-19}{8}=\dfrac{16}{8}=2\)

`2,`

`a)`

\(\dfrac{3}{7}+\dfrac{5}{4}=\dfrac{12}{28}+\dfrac{35}{28}=\dfrac{12+35}{28}=\dfrac{47}{28}\)

`b)`

\(\dfrac{4}{5}+\dfrac{12}{10}=\dfrac{8}{10}+\dfrac{12}{10}=\dfrac{8+12}{10}=\dfrac{20}{10}=2\)

`c)`

\(\dfrac{12}{8}-\dfrac{4}{9}=\dfrac{108}{72}-\dfrac{32}{72}=\dfrac{76}{72}=\dfrac{19}{18}\)

`d)`

\(\dfrac{2}{7}-\dfrac{3}{14}=\dfrac{4}{14}-\dfrac{3}{14}=\dfrac{1}{14}\)

Thầy cô hay các bạn giúp mk câu này với!

Câu 6: \(\dfrac{40}{7}\) \(\times\) \(\dfrac{14}{5}\) = \(\dfrac{40}{5}\) \(\times\) \(\dfrac{14}{7}\) = 8 \(\times\) 2 = 16

Chọn C.16

Câu 7: 2\(\dfrac{1}{4}\) = \(\dfrac{2\times4+1}{4}\) = \(\dfrac{9}{4}\); 

        2\(\dfrac{1}{4}\) gấp \(\dfrac{1}{8}\) số lần là: \(\dfrac{9}{4}\) : \(\dfrac{1}{8}\) = 18 (lần)

Chọn B. 18

Câu 8: Chiều rộng của hình chữ nhật là: \(\dfrac{25}{12}\) : \(\dfrac{5}{3}\) = \(\dfrac{5}{4}\) (m)

Chu vi của hình chữ nhật là: ( \(\dfrac{5}{3}\) + \(\dfrac{5}{4}\))  \(\times\) 2 = \(\dfrac{35}{6}\)(m)

                   \(\dfrac{35}{6}\) m = 5\(\dfrac{5}{6}\) m 

Chọn B. 5\(\dfrac{5}{6}\) m

9876

9876

\(a.\dfrac{7}{19}+\dfrac{12}{19}=\dfrac{7+12}{19}=\dfrac{19}{19}=1\)

\(b.\dfrac{5}{13}+\dfrac{4}{13}=\dfrac{5+4}{13}=\dfrac{9}{13}\)

\(c.\dfrac{24}{11}-\dfrac{13}{11}=\dfrac{24-13}{11}=\dfrac{11}{11}=1\)

\(d.\dfrac{35}{8}-\dfrac{19}{8}=\dfrac{35-19}{8}=\dfrac{16}{8}=2\)

a,\(\dfrac{7}{19}\)+\(\dfrac{12}{19}\)=\(\dfrac{19}{19}\)=1

b,\(\dfrac{5}{13}\)+\(\dfrac{4}{13}\)=\(\dfrac{9}{13}\)

c,\(\dfrac{24}{11}\)-\(\dfrac{13}{11}\)=\(\dfrac{11}{11}\)=1

d,\(\dfrac{35}{8}\)-\(\dfrac{19}{8}\)=\(\dfrac{16}{8}\)=2

3\(\dfrac{1}{5}\)=3+\(\dfrac{1}{5}\)=\(\dfrac{16}{5}\)

C

Ta có:

\(\dfrac{4}{12}=\dfrac{1}{3}\)

\(\dfrac{5}{20}=\dfrac{1}{4}\)

\(\dfrac{21}{35}=\dfrac{3}{5}\)

\(\dfrac{17}{34}=\dfrac{1}{2}\)

Vậy \(\dfrac{17}{34}=\dfrac{1}{2}\)

không có phân số nào

Bài 1: 

$|x|=\frac{1}{3}\Rightarrow x=\frac{1}{3}$ hoặc $x=\frac{-1}{3}$

Nếu $x=\frac{1}{3}$ thì $A=3x^2-4x+5=3.(\frac{1}{3})^2-4.\frac{1}{3}+5=4$

Nếu $x=\frac{-1}{3}$ thì $A=3x^2-4x+5=3.(\frac{-1}{3})^2-4.\frac{-1}{3}+5=\frac{20}{3}$

Baif 2: 

$|x|=2\Rightarrow x=\pm 2$

Nếu $x=2$ thì $B=-2x^2+3x-2=-2.2^2+3.2-2=-4$

Nếu $x=-2$ thì $B=-2x^2+3x-2=-2.(-2)^2+3(-2)-2=-16$

Bài 3:

Nếu $x\geq 5$ thì:

$C=2(5x+2)-(x-5)=10x+4-x+5=9x+9$

Nếu $x<5$ thì:

$C=2(5x+2)-(5-x)=10x+4-5+x=11x-1$