a: \(A=\dfrac{15x^5y^3}{5x^2y^2}-\dfrac{10x^3y^2}{5x^2y^2}+\dfrac{20x^4y^4}{5x^2y^2}\)
\(=3x^3y-2x+4x^2y^2\)
Khi x=-1 và y=2 thì \(A=3\cdot\left(-1\right)^3\cdot2-2\cdot\left(-1\right)+4\cdot\left(-1\cdot2\right)^2\)
\(=-6+2+4\cdot4=16+2-6=12\)
b: \(B=\dfrac{4x^4y^2}{x^2y^2}+\dfrac{3x^4y^3}{x^2y^2}-\dfrac{6x^3y^2}{x^2y^2}\)
\(=4x^2+3x^2y-6x\)
Khi x=y=-2 thì \(B=4\cdot\left(-2\right)^2+3\cdot\left(-2\right)^2\cdot\left(-2\right)-6\cdot\left(-2\right)\)
\(=4\cdot4-6\cdot4+12\)
=16+12-24
=16-12
=4
c: \(C=\dfrac{-2x^2y^2}{\dfrac{2}{3}xy}+\dfrac{4xy}{\dfrac{2}{3}xy}-\dfrac{6xy^3}{\dfrac{2}{3}xy}\)
\(=-3xy+6-9y^2\)
Khi x=1/2 và y=4 thì \(C=-3\cdot\dfrac{1}{2}\cdot4+6-9\cdot4^2=-144\)
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