1 cây bút: 50.000:5=10.000(đồng)

18 cây bút: 10.000x18=180.000(đồng)

Vậy 18 cây bút có giá 180.000 đồng

12/5

4/6 + 7/12 + 17/9 + 1/9 + 14/16
= 2/3 + 7/12 + 17/9 + 1/9 + 7/8
= 8/12 + 7/12 + (17/9 + 1/9) + 7/8
= 15/12 + 18/9 + 7/8
= 5/4 + 2 + 7/8
= 10/8 + 16/8 + 1/8
= 10+16+1/8
= 27/8

Tử số là (18-8):2=10:2=5

Mẫu số là 5+8=13

Vậy: Phân số cần tìm là \(\dfrac{5}{13}\)

Tử số là:

(18-8):2=5

Mẫu số là:

5+8=13

 vậy phân số đó là:5/13

Trong 2 tuần nhà An đã dùng hết:

\(500\times\dfrac{2}{5}=200\left(ml\right)\)

Dùng hết số ml là : \(500x\dfrac{2}{5}\)=200ml

12 / 17 ; 133/143;7/7;3/2;21/8

\(\dfrac{2\times3\times4\times6}{2\times3\times4\times8}=\dfrac{2}{2}\times\dfrac{3}{3}\times\dfrac{4}{4}\times\dfrac{6}{8}\)

\(=1\times1\times1\times\dfrac{3}{4}=\dfrac{3}{4}\)

\(\dfrac{2\times3\times4\times6}{2\times3\times4\times8}=\dfrac{6}{8}=\dfrac{3}{4}\)

\(\dfrac{1}{2}+\dfrac{1}{6}+...+\dfrac{1}{90}\)

\(=\dfrac{1}{1\times2}+\dfrac{1}{2\times3}+...+\dfrac{1}{9\times10}\)

\(=1-\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+...+\dfrac{1}{9}-\dfrac{1}{10}\)

\(=1-\dfrac{1}{10}=\dfrac{9}{10}\)

\(\dfrac{1}{2}+\dfrac{1}{6}+\dfrac{1}{12}+\dfrac{1}{20}+...+\dfrac{1}{90}\)

\(=\dfrac{1}{1\times2}+\dfrac{1}{2\times3}+\dfrac{1}{3\times4}+\dfrac{1}{4\times5}+...+\dfrac{1}{9\times10}\)

\(=1-\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{5}+...+\dfrac{1}{9}-\dfrac{1}{10}\)

\(=1-\dfrac{1}{10}\)

\(=\dfrac{10}{10}-\dfrac{1}{10}\)

\(=\dfrac{9}{10}\)

a: \(\dfrac{4}{6}+\dfrac{7}{13}+17+9+19+18+\dfrac{1}{9}+\dfrac{14}{6}\)

\(=\left(\dfrac{4}{6}+\dfrac{14}{6}\right)+\dfrac{1}{9}+\dfrac{7}{13}+63\)

\(=3+63+\dfrac{13+63}{117}\)

\(=66+\dfrac{76}{117}=\dfrac{7798}{117}\)

b: \(\dfrac{2}{5}\times\dfrac{1}{4}+\dfrac{3}{4}\times\dfrac{2}{5}\)

\(=\dfrac{2}{5}\times\left(\dfrac{1}{4}+\dfrac{3}{4}\right)=\dfrac{2}{5}\times1=\dfrac{2}{5}\)

b) \(\dfrac{2}{5}\times\dfrac{1}{4}+\dfrac{3}{4}\times\dfrac{2}{5}=\dfrac{2}{5}\times\left(\dfrac{1}{4}+\dfrac{3}{4}\right)=\dfrac{2}{5}\times1=\dfrac{2}{5}\)

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