Bài 1:
\(3\dfrac{4}{5}=\dfrac{19}{5};8\dfrac{7}{10}=\dfrac{87}{10};2\dfrac{3}{4}=\dfrac{11}{4};9\dfrac{1}{2}=\dfrac{19}{2}\)
\(25\dfrac{15}{17}=\dfrac{440}{17};18\dfrac{27}{40}=\dfrac{747}{40};2\dfrac{1}{2}=\dfrac{5}{2};6\dfrac{2}{5}=\dfrac{32}{5};3\dfrac{1}{4}=\dfrac{13}{4}\)
`3/x = 12/16`
`=> 3 : x = 12/16`
` x = 3 : 12/16`
` x= 3 xx 16/12`
` x = 4`
Vậy ` x = 4`
Mai mốt đăng từng bài một thôi nha cậu. Nhìn mỏi cổ đau mắt quá :)
Bài 1:
\(3\dfrac{4}{5}=\dfrac{3\times5+4}{5}=\dfrac{19}{5}\\ 8\dfrac{7}{10}=\dfrac{8\times10+7}{10}=\dfrac{87}{10}\\ 2\dfrac{3}{4}=\dfrac{2\times4+3}{4}=\dfrac{11}{4}\\ 9\dfrac{1}{2}=\dfrac{9\times2+1}{2}=\dfrac{19}{2}\\ 25\dfrac{15}{17}=\dfrac{25\times17+15}{17}=\dfrac{440}{17}\\ 18\dfrac{27}{40}=\dfrac{18\times40+27}{40}=\dfrac{747}{40}\\ 2\dfrac{1}{2}=\dfrac{2\times2+1}{2}=\dfrac{5}{2}\\ 6\dfrac{2}{5}=\dfrac{6\times5+2}{5}=\dfrac{32}{5}\\ 3\dfrac{1}{4}=\dfrac{3\times4+1}{4}=\dfrac{13}{4}\)
Bài 2:
\(a)2\dfrac{1}{5}+3\dfrac{2}{5}=\dfrac{11}{5}+\dfrac{17}{5}=\dfrac{11+17}{5}=\dfrac{28}{5}\\ b)1\dfrac{2}{7}+5\dfrac{3}{7}=\dfrac{9}{7}+\dfrac{38}{7}=\dfrac{9+28}{7}=\dfrac{37}{7}\\ c)2\dfrac{7}{8}-1\dfrac{3}{8}=\dfrac{23}{8}-\dfrac{11}{8}=\dfrac{23-11}{8}=\dfrac{12}{8}=\dfrac{3}{2}\\ d)8\dfrac{1}{4}-5\dfrac{3}{4}=\dfrac{33}{4}-\dfrac{23}{4}=\dfrac{33-23}{4}=\dfrac{10}{4}=\dfrac{5}{2}\\ e)3\dfrac{1}{3}\times2\dfrac{4}{5}=\dfrac{10}{3}\times\dfrac{14}{5}=\dfrac{10\times14}{3\times5}=\dfrac{140}{15}=\dfrac{28}{3}\\ f)4\dfrac{1}{2}:2\dfrac{1}{7}=\dfrac{9}{2}:\dfrac{15}{7}=\dfrac{9}{2}\times\dfrac{7}{15}=\dfrac{9\times7}{2\times15}=\dfrac{63}{30}=\dfrac{21}{10}\)
Bài 3:
\(a)\left(5\dfrac{3}{8}+18\dfrac{1}{2}-7\dfrac{5}{24}\right):16\dfrac{2}{3}=\left(\dfrac{43}{8}+\dfrac{37}{2}-\dfrac{173}{24}\right):\dfrac{50}{3}=\left(\dfrac{129}{24}+\dfrac{444}{24}-\dfrac{173}{24}\right):\dfrac{50}{3}=\left(\dfrac{129+444-173}{24}\right):\dfrac{50}{3}=\dfrac{400}{24}:\dfrac{50}{3}=\dfrac{50}{3}:\dfrac{50}{3}=\dfrac{50:50}{3:3}=\dfrac{1}{1}=1\)
\(b)3\dfrac{1}{2}+4\dfrac{5}{7}-5\dfrac{5}{4}=\dfrac{7}{2}+\dfrac{33}{7}-\dfrac{25}{4}=\dfrac{7\times14}{2\times14}+\dfrac{33\times4}{7\times4}-\dfrac{25\times7}{4\times7}=\dfrac{98}{28}+\dfrac{132}{28}-\dfrac{175}{28}=\dfrac{98+132-175}{28}=\dfrac{55}{28}\)
\(c)4\dfrac{1}{2}+\dfrac{1}{2}:5\dfrac{5}{4}=\dfrac{9}{2}+\dfrac{1}{2}:\dfrac{25}{4}=\dfrac{9}{2}+\dfrac{1}{2}\times\dfrac{4}{25}=\dfrac{9}{2}+\dfrac{1\times4}{2\times25}=\dfrac{9}{2}+\dfrac{4}{50}=\dfrac{9}{2}+\dfrac{2}{25}=\dfrac{9\times25}{2\times25}+\dfrac{2\times2}{25\times2}=\dfrac{225}{50}+\dfrac{4}{50}=\dfrac{225+4}{50}=\dfrac{229}{50}\)
\(d)7\dfrac{1}{6}-2\dfrac{2}{3}-3\dfrac{1}{2}=\dfrac{43}{6}-\dfrac{8}{3}-\dfrac{7}{2}=\dfrac{43}{6}-\dfrac{8\times2}{3\times2}-\dfrac{7\times3}{2\times3}=\dfrac{43}{6}-\dfrac{16}{6}-\dfrac{21}{6}=\dfrac{43-16-21}{6}=\dfrac{6}{6}=1\)
\(e)4\dfrac{4}{9}-\left(2\dfrac{1}{4}+1\dfrac{4}{9}\right)=\dfrac{40}{9}-\left(\dfrac{9}{4}+\dfrac{13}{9}\right)=\dfrac{40}{9}-\left(\dfrac{9\times9}{4\times9}+\dfrac{13\times4}{9\times4}\right)=\dfrac{40}{9}-\left(\dfrac{81}{36}+\dfrac{52}{36}\right)=\dfrac{40}{9}-\left(\dfrac{81+52}{36}\right)=\dfrac{40}{9}-\dfrac{133}{36}=\dfrac{40\times4}{9\times4}-\dfrac{133}{36}=\dfrac{160}{36}-\dfrac{133}{36}=\dfrac{160-133}{36}=\dfrac{27}{36}=\dfrac{3}{4}\)
Bài 4:
\(a)\dfrac{3}{x}=\dfrac{12}{16}\)
\(3:x=\dfrac{12}{16}\)
\(x=3:\dfrac{12}{16}\)
\(x=3\times\dfrac{16}{12}\)
\(\Rightarrow x=4\)
\(b)\dfrac{8}{15}=\dfrac{2}{x}\\ x=2:\dfrac{8}{15}\\ x=2\times\dfrac{15}{8}\\ x=\dfrac{15}{4}\)