Câu 1:

Thay \(x=-12\) vào \(\left|x-2\right|\)

\(\Rightarrow\left|-12-2\right|=\left|-14\right|=14\)

Câu 2: Chọn phương án A.

Câu 3:

\(\left|-120\right|+\left|20\right|=120+20=140\)

\(\dfrac{1}{4}+\dfrac{3}{7}+\dfrac{11}{14}=\dfrac{7}{28}+\dfrac{12}{28}+\dfrac{22}{28}=\dfrac{7+12+22}{28}=\dfrac{41}{28}\)

a) \(\dfrac{-2}{9}=-\dfrac{2}{9}\) ; \(\dfrac{6}{-27}=\dfrac{2}{-9}=-\dfrac{2}{9}\)

Vậy \(\dfrac{-2}{9}=\dfrac{6}{-27}\).

b) \(\dfrac{-1}{-5}=\dfrac{1}{5}=\dfrac{5}{25}\) ; \(\dfrac{4}{25}\)

Vậy hai phân số này không bằng nhau.

Rút gọn \(\dfrac{100}{8}=\dfrac{25}{2}\)

\(\dfrac{5}{2}\times\dfrac{1}{3}+\dfrac{1}{4}=\dfrac{5}{6}+\dfrac{1}{4}=\dfrac{10}{12}+\dfrac{3}{12}=\dfrac{13}{12}\)

\(-5\left(x+4\right)-3\left(x-2\right)=-114\)

\(\Leftrightarrow-5x-20-3x+6=-114\)

\(\Leftrightarrow-5x-3x=-114+20-6\)

\(\Leftrightarrow-8x=-100\)

\(\Leftrightarrow x=\dfrac{-100}{-8}=\dfrac{100}{8}\)

Vì 1 dm² = 100 cm²

\(\dfrac{2}{5}dm^2=0,4dm^2=40cm^2\)

Vì 1 dm² = 100 cm²

\(4dm^212cm^2=412cm^2\)

Vì \(1m^2=100dm^2\) 

\(\dfrac{3}{4}m^2=0,75m^2=75dm^2\)

\(\dfrac{9}{x+4}=\dfrac{3}{x-2}\) (ĐK: \(x\ne2;-4\))

\(\Rightarrow9\left(x-2\right)=3\left(x+4\right)\)

\(\Leftrightarrow9x-18=3x+12\)

\(\Leftrightarrow9x-3x=12+18\)

\(\Leftrightarrow6x=30\)

\(\Leftrightarrow x=5\) (TMĐK)

Vậy \(S=\left\{5\right\}\).