a: \(A=4x^3y-xy-\dfrac{9}{2}x^3y+3xy-1\)
\(=4x^3y-\dfrac{9}{2}x^3y+3xy-xy-1\)
\(=-\dfrac{1}{2}x^3y+2xy-1\)
Thay x=-1 và y=1 vào A, ta được:
\(A=\dfrac{-1}{2}\cdot\left(-1\right)^3\cdot1+2\cdot\left(-1\right)\cdot1-1\)
\(=\dfrac{1}{2}-2-1=\dfrac{1}{2}-3=-\dfrac{5}{2}\)
b: \(B=\dfrac{3}{4}xy^2-\dfrac{1}{3}x^2y-\dfrac{5}{6}xy^2+2x^2y\)
\(=\left(\dfrac{3}{4}xy^2-\dfrac{5}{6}xy^2\right)+\left(2x^2y-\dfrac{1}{3}x^2y\right)\)
\(=\dfrac{-1}{12}xy^2+\dfrac{5}{3}x^2y\)
Thay x=-1 và y=1 vào B, ta được:
\(B=-\dfrac{1}{12}\cdot\left(-1\right)\cdot1^2+\dfrac{5}{3}\left(-1\right)^2\cdot1\)
\(=\dfrac{1}{12}+\dfrac{5}{3}=\dfrac{1}{12}+\dfrac{20}{12}=\dfrac{21}{12}=\dfrac{7}{4}\)
