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

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

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

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

vì sao vinh dự

ko bt hôm nay ngày j à

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

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

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

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

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

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

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

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

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

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

=167,132

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

2 lần vận tốc thật của cano là:

\(6\times5+8\times5=70\left(\dfrac{km}{h}\right)\)

vận tốc thật của cano là:

70:2=35(km/h)

Độ dài quãng sông AB là:

8x(35-5)=8x30=240(km)

\(\dfrac{3}{13}-\left(\dfrac{4}{7}+\dfrac{3}{13}\right)=\dfrac{3}{13}-\dfrac{4}{7}-\dfrac{3}{13}=\left(\dfrac{3}{13}-\dfrac{3}{13}\right)-\dfrac{4}{7}=0-\dfrac{4}{7}=-\dfrac{4}{7}\)

= 3/13 - 4/7 - 3/13
= (3/13 - 3/13) - 4/7
=0- 4/7
= -4/7