a, \(\dfrac{15\times14-1}{13\times15+14}\)

 = \(\dfrac{15\times\left(13+1\right)-1}{13\times15+14}\) 

= \(\dfrac{13\times15+15-1}{13\times15+14}\) 

= \(\dfrac{13\times15+14}{13\times15+14}\)

= 1

c, \(\dfrac{1+2+3+4+5+6+7+8+9}{10}\)

= \(\dfrac{\left(9+1\right)\times\left\{\left(9-1\right):1+1\right\}:2}{10}\)

= \(\dfrac{10\times9:2}{10}\)

= \(\dfrac{9}{2}\)

\(\left(\dfrac{5}{6}+\dfrac{7}{x}\right)-\dfrac{16}{12}=\dfrac{2}{3}\)

\(\Rightarrow\dfrac{5}{6}+\dfrac{7}{x}=\dfrac{2}{3}+\dfrac{16}{12}=\dfrac{8}{12}+\dfrac{16}{12}=\dfrac{24}{12}=2\)

\(\Rightarrow\dfrac{7}{x}=2-\dfrac{5}{6}=\dfrac{12}{6}-\dfrac{5}{6}=\dfrac{7}{6}\)

=> x = 6

(\(\dfrac{5}{6}\) + \(\dfrac{7}{x}\)) - \(\dfrac{16}{12}\) = \(\dfrac{2}{3}\)

\(\dfrac{5}{6}\) + \(\dfrac{7}{x}\) = \(\dfrac{2}{3}\) + \(\dfrac{16}{12}\)

\(\dfrac{5}{6}\) + \(\dfrac{7}{x}\) = 2

\(\dfrac{7}{x}\) = 2 - \(\dfrac{5}{6}\)

\(\dfrac{7}{x}\) = \(\dfrac{1}{6}\)

\(x\) = 6 x (7 : 1)

\(x\) = 6 x 7

\(x\) = 42

#Yu

a)x*4/5:2=6/7               b)6/5:x:5/4=10/15

x*4/5=6/7 x 2                6/5:x=10/15 x 5/4

x*4/5=12/7                     6/5:x=5/6

x=12/7:4/5                       x=6/5:5/6

x=15/7                            x=36/25

a) x * 4/5 : 2 = 6/7

x * 4/5 = 6/7 *2

x *4/5        = 12/7

x                = 12/7 : 4/5

x               = 15/7

    Vậy x = 15/7

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