a) 

Do `3/2 > 1; 4/3 > 1; 7/6 > 1`

`=> 3/2 + 4/3 + 7/6 > 1+1+1`

`=> 3/2 + 4/3 + 7/6 > 3`

b) 

Do `4/7 < 1; 5/12 <1; 84/87 < 1`

`=> 4/7 + 5/12 + 84/87 < 1+1+1`

`=> 4/7 + 5/12 + 84/87 < 3`

a. \(\frac{3}{4}\times x+\frac{5}{6}\times x-\frac{5}{2}=7\)

\(x\times\left(\frac{3}{4}+\frac{5}{6}\right)=7+\frac{5}{2}\)

\(x\times\left(\frac{9}{12}+\frac{10}{12}\right)=\frac{14}{2}+\frac{5}{2}\)

\(x\times\frac{19}{12}=\frac{19}{2}\)

\(x=\frac{19}{2}\div\frac{19}{12}\)

\(x=\frac{19}{2}\times\frac{12}{19}\)

\(x=6\)

b) \(12\times x-12\times84=0\)

\(12\times x=12\times84\)

\(x=84\)

a) \(\dfrac{3}{4}\times x:\dfrac{1}{2}=\dfrac{4}{5}\)

\(\dfrac{3}{4}\times x=\dfrac{4}{5}\times\dfrac{1}{2}\)

\(\dfrac{3}{4}\times x=\dfrac{2}{5}\)

\(x=\dfrac{2}{5}:\dfrac{3}{4}\)

\(x=\dfrac{8}{15}\)

b) \(\left(14-x:6\right)\times7=84\)

\(14-x:6=84:7\)

\(14-x:6=12\)

\(x:6=14-12\)

\(x:6=2\)

\(x=6\times2=12\)

c) \(x\times1,5:2,4=0,3\)

\(x\times1,5=0,3\times2,4\)

\(x\times1,5=0,72\)

\(x=0,72:1,5\)

\(x=0,48\)

d) Thiếu x

\(a,7\left(x-4\right)-3\left(x+8\right)=-84\)

\(7x-29-3x+24=-8\)

\(7x-3x=-8+29-24\)

\(4x=-3\Leftrightarrow x=-\frac{3}{4}\)

\(b,3\left(x-8\right)-6\left(x-5\right)=12\)

\(3x-24-6x-30=12\)

\(3x-6x=12+24+30\)

\(-3x=66\Leftrightarrow x=-22\)

\(c,3x-7⋮x-3\)

\(3\left(x-3\right)+2⋮x-3\)

Vì \(3\left(x-3\right)⋮x-3\)

\(\Rightarrow2⋮x-3\)

\(\Rightarrow x-3\inƯ\left(2\right)=\left\{\pm1;\pm2\right\}\)

Ta lập bảng 

a) 7.(x-4)-3.(x+8)=-84

    7x-28-3x-24=-84

    7x-3x=-84+28+24

    4x=-32

      x=-32:4

      x=-9

Vậy x=-9

b) 3.(x-8)-6.(x-5)=12

    3x-24-6x+30=12

    3x-6x=12+24-30

    -3x=6

       x=6:(-3)

       x=-2

Vậy x=-2

\(9+2+5+3+7+8+4+2+9+6+4+65+6+7+4+6+3+7+4+8+5+9+08+87+1+2+3+4+2+3+2+5+6+767+5+75+6+4+6+5+66+5+7+546+46+43+6+3+6+4+6+7+8878+68+68+6+7+9+9=10961\)

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