\(A=4\left(x^2y-2xy^2+\dfrac{1}{2}\right)-2x\left(xy-4y^2\right)+\dfrac{1}{2}\\ A=4x^2y-8xy^2+2-2x^2y+8xy^2+\dfrac{1}{2}\\ A=\left(4x^2y-2x^2y\right)-\left(8xy^2-8xy^2\right)+\left(2+\dfrac{1}{2}\right)\\ A=2x^2y+\dfrac{5}{2}\\ Thayx=-1,tacó;\\ A=2\cdot\left(-1\right)^2\cdot y+\dfrac{5}{2}\\ =2y+\dfrac{5}{2}\)
Vậy A= \(2y+\dfrac{5}{2}\) khi x = -1
\(A=4.\left(x^2y-2xy^2+\dfrac{1}{2}\right)-2x\left(xy-4y^2+\dfrac{1}{2}\right)\)
\(A=4x^2y-8xy^2+2-2x^2y+8xy^2+\dfrac{1}{2}\)
\(A=\left(4x^2y-2x^2y\right)+\left(-8xy^2+8xy^2\right)+\left(2+\dfrac{1}{2}\right)\)
\(A=2x^2y+\dfrac{5}{2}\)
Với x = -1
\(\Rightarrow A=2.\left(-1\right)^2.y+\dfrac{5}{2}\)
\(A=2.1.y+\dfrac{5}{2}\)
\(A=2y+\dfrac{5}{2}\)
Vậy x = -1 thì \(A=2y+\dfrac{5}{2}\)