a: 1,2m=12dm

Diện tích xung quanh bể cá là:

\(\left(12+8\right)\cdot2\cdot6=12\cdot20=240\left(dm^2\right)\)

Diện tích kính dùng làm bể cá là:

\(240+12\cdot8=336\left(dm^2\right)\)

b: Thể tích bể cá là:

\(12\cdot8\cdot6=48\cdot12=576\left(dm^3\right)\)

c: Thể tích nước trong bể là:

\(576\cdot\dfrac{2}{3}=384\left(lít\right)\)

16: \(0\cdot\dfrac{195}{2007}=0\)

1: \(\dfrac{4}{5}\cdot\dfrac{1}{2}=\dfrac{4\cdot1}{5\cdot2}=\dfrac{4}{10}=\dfrac{2}{5}\)

2: \(\dfrac{3}{7}\cdot\dfrac{3}{5}=\dfrac{3\cdot3}{5\cdot7}=\dfrac{9}{35}\)

3: \(\dfrac{1}{2}\cdot\dfrac{2}{9}=\dfrac{1\cdot2}{2\cdot9}=\dfrac{2}{18}=\dfrac{1}{9}\)

4: \(\dfrac{1}{5}\cdot\dfrac{3}{8}=\dfrac{1\cdot3}{5\cdot8}=\dfrac{3}{40}\)

5: \(7\cdot\dfrac{6}{7}=\dfrac{7\cdot6}{7}=\dfrac{42}{7}=6\)

6: \(5\cdot\dfrac{15}{4}=\dfrac{5\cdot15}{4}=\dfrac{75}{4}\)

7: \(\dfrac{4}{8}\cdot\dfrac{5}{3}=\dfrac{4\cdot5}{8\cdot3}=\dfrac{20}{24}=\dfrac{5}{6}\)

8: \(\dfrac{4}{9}\cdot\dfrac{2}{3}=\dfrac{4\cdot2}{9\cdot3}=\dfrac{8}{27}\)

9: \(\dfrac{5}{6}\cdot\dfrac{4}{7}=\dfrac{5\cdot4}{6\cdot7}=\dfrac{20}{42}=\dfrac{5}{21}\)

10: \(\dfrac{7}{9}\cdot\dfrac{3}{5}=\dfrac{7\cdot3}{5\cdot9}=\dfrac{21}{45}=\dfrac{7}{15}\)

11: \(9\cdot\dfrac{5}{6}=\dfrac{9\cdot5}{6}=\dfrac{45}{6}=\dfrac{15}{2}\)

12: \(\dfrac{2}{5}\cdot15=\dfrac{2\cdot15}{5}=\dfrac{30}{5}=6\)

13: \(\dfrac{5}{6}\cdot\dfrac{7}{8}=\dfrac{5\cdot7}{6\cdot8}=\dfrac{35}{48}\)

14: \(4\cdot\dfrac{3}{16}=\dfrac{4\cdot3}{16}=\dfrac{12}{16}=\dfrac{3}{4}\)

15: \(\dfrac{7}{3}\cdot\dfrac{3}{2}=\dfrac{7\cdot3}{3\cdot2}=\dfrac{21}{6}=\dfrac{7}{2}\)

17: \(\dfrac{15}{3}\cdot\dfrac{3}{5}=\dfrac{15\cdot3}{3\cdot5}=\dfrac{45}{15}=3\)

18: \(\dfrac{7}{5}\cdot\dfrac{15}{14}=\dfrac{7\cdot15}{5\cdot14}=\dfrac{105}{70}=\dfrac{3}{2}\)

19: \(20\cdot\dfrac{7}{4}=\dfrac{20\cdot7}{4}=\dfrac{140}{4}=35\)

20: \(\dfrac{11}{3}\cdot\dfrac{26}{7}=\dfrac{11\cdot26}{3\cdot7}=\dfrac{286}{21}\)

`630 : x - 24 = 18`

`630 : x = 18 + 24`

`630 : x = 42`

`x = 630 : 42`

`x = 15`

630:x-24=18

=>630:x=24+18=42

=>x=630:42=15

de bai sai a ban

sao chieu dai lai bang chieu rong duoc

Ta có:

`4x-5y-6xy+7=0`

`⇒(4x-6xy)-5y+7=0`

`⇒2x(2-3y)-5y+7=0`

`⇒6x(2-3y)-15y+21=0` 

`⇒6x(2-3y)+21-15y=0` 

`⇒6x(2-3y)+(10-15y)+11=0`

`⇒-6x(3y-2)-(15y-10)+11=0`

`⇒-6x(3y-2)-5(3y-2)=-11`

`⇒(3y-2)(6x+5)=11` 

Mà `x,y∈Z` 

Ta có bảng: 

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