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Quy đồng mẫu hai phân số ta có :

\(\frac{4}{7}=\frac{4\times12}{7\times12}=\frac{48}{84}\)

\(\frac{5}{12}=\frac{5\times7}{12\times7}=\frac{35}{84}\)

Giải:

\(\dfrac{3}{2}+\dfrac{6}{8}+\dfrac{9}{28}+\dfrac{12}{77}+\dfrac{15}{176}\) 

\(=\dfrac{3}{1.2}+\dfrac{6}{2.4}+\dfrac{9}{4.7}+\dfrac{12}{7.11}+\dfrac{15}{11.16}\) 

\(=3.\left(\dfrac{1}{1.2}+\dfrac{2}{2.4}+\dfrac{3}{4.7}+\dfrac{4}{7.11}+\dfrac{5}{11.16}\right)\) 

\(=3.\left(1-\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{11}+\dfrac{1}{11}-\dfrac{1}{16}\right)\) 

\(=3.\left(1-\dfrac{1}{16}\right)\) 

\(=3.\dfrac{15}{16}\) 

\(=\dfrac{45}{16}\)

Đặt A = \(\dfrac{3}{2}+\dfrac{6}{8}+\dfrac{9}{28}+\dfrac{12}{77}+\dfrac{15}{176}\)

= \(3\left(\dfrac{1}{1\times2}+\dfrac{2}{2\times4}+\dfrac{3}{4\times7}+\dfrac{4}{7\times11}+\dfrac{5}{11\times16}\right)\)

= \(3\left(1-\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{11}+\dfrac{1}{11}-\dfrac{1}{16}\right)\)

= \(3\left(1-\dfrac{1}{16}\right)=3\times\dfrac{15}{16}=\dfrac{45}{16}\)

\(\frac{5}{4}+\frac{7}{12}+\frac{7}{5}+\frac{75}{100}+\frac{40}{25}+\frac{20}{48}\)

\(=\frac{5}{4}+\frac{7}{12}+\frac{7}{5}+\frac{3}{4}+\frac{8}{5}+\frac{5}{12}\)

\(=\left(\frac{5}{4}+\frac{3}{4}\right)+\left(\frac{7}{12}+\frac{5}{12}\right)+\left(\frac{7}{5}+\frac{8}{5}\right)\)

\(=\frac{8}{4}+\frac{12}{12}+\frac{15}{5}\)

\(=2+1+3\)

\(=6\)

Đặt A=5/4 + 7/12 + 7/5 + 75/100 + 40/25 + 20/48

=>A=125/100 + 28/48 + 35/25 + 75/100 + 40/25 + 20/48

=>A=(125/100 + 75/100) + (28/48 + 20/48) + (35/25 + 40/25)

=>A=        2                     +            1             +           3

=>A=                    6

Vậy A=6

A)\(\frac{3}{5}=\frac{42}{70};\frac{4}{7}=\frac{40}{70};\frac{5}{10}=\frac{35}{70}\)

B)\(\frac{4}{9}=\frac{8}{18};\frac{11}{18}=\frac{11}{18};\frac{5}{6}=\frac{15}{18}\)

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