82

B

\(2x^2-4xy+2y^2\\ =2\left(x^2-2xy+y^2\right)\\ =2\left(x-y\right)^2\)

\(\dfrac{2-x}{4}=\dfrac{3x-1}{3}\\ \Rightarrow3\left(2-x\right)=4\left(3x-1\right)\\ \Rightarrow6-3x=12x-4\\ \Rightarrow10=15x\\ \Rightarrow x=\dfrac{2}{3}\)

D

\(126-x:5=42\\ x:5=84\\ x=420\)

\(x+y+z=0\\ \Rightarrow x+y=-z\\ \Rightarrow\left(x+y\right)^3=\left(-z\right)^3\\ \Rightarrow x^3+3x^2y+3xy^2+y^3\\ \Rightarrow x^2+y^2+z^2=-3x^2y-3xy^2\\ \Rightarrow x^2+y^2+z^2=-3xy\left(x+y\right)\\ \Rightarrow x^2+y^2+z^2=-3xy\left(-z\right)=3xyz\\ \left(đpcm\right)\)

C

\(=56+710+142\\ =908\)

\(\left(x-1\right)\left(x+2\right)-x-2=0\\ x^2+2x-x-2-x-2=0\\ x^2-4=0\\ x^2=4\\ x=2\)