a/ \(A=3x^2-2\left(x-y\right)^2-3y^2\)
\(=3\left(x^2-y^2\right)-2\left(x-y\right)^2\)
\(=3\left(x-y\right)\left(x+y\right)-2\left(x-y\right)^2\)
\(=\left(x-y\right)\left(3x+3y-2x+2y\right)\)
\(=\left(x-y\right)\left(x+5y\right)\)
Thay \(x=4;y=-4\) vào biểu thức ta có :
\(A=\left(4-\left(-4\right)\right)\left(4+5.\left(-4\right)\right)=-128\)
Vậy...
b/ \(B=4\left(x-2\right)\left(x+1\right)+\left(2x-4\right)^2+\left(x+1\right)^2\)
\(=\left(x+1\right)^2+4\left(x-2\right)\left(x+1\right)+4\left(x-2\right)^2\)
\(=\left(x+1+2x-4\right)^2\)
\(=\left(3x-3\right)^2\)
Thay \(x=\frac{1}{2}\) vào biểu thức B ta có :
\(A=\left(3.\frac{1}{2}-3\right)^2=\frac{9}{4}\)
Vậy..
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