a: \(P=7x\left(x^2y-5xy^2+2y^3\right)\)\(=7x\cdot x^2y-7x\cdot5xy^2+7x\cdot2y^3\)\(=7x^3y-35x^2y^2+14xy^3\)\(M=5y\left(x^2-3xy\right)\left(x-4y\right)\)\(=5y\left(x^3-4x^2y-3x^2y+12xy^2\right)\)\(=5y\cdot x^3-5y\cdot7x^2y+5y\cdot12xy^2\)\(=5x^3y-35x^2y^2+60xy^3\)\(N=\left(\dfrac{1}{2}x^4y^2-10x^2y^4\right):\left(-\dfrac{1}{4}xy\right)\)\(=-\dfrac{1}{2}x^4y^2:\dfrac{1}{4}xy+10x^2y^4:\dfrac{1}{4}xy\)\(=-2x^3y+40xy^3\)b: A=P-M-N\(=7x^3y-35x^2y^2+14xy^3-5x^3y+35x^2y^2-60xy^3+2x^3y-40xy^3\)\(=\left(7x^3y-5x^3y+2x^3y\right)+\left(-35x^2y^2+35x^2y^2\right)+\left(14xy^3-40xy^3-60xy^3\right)\)=\(4x^3y-86xy^3\)Thay x=-3;y=-1 vào A, ta được:\(A=4\cdot\left(-3\right)^3\cdot\left(-1\right)-86\cdot\left(-3\right)\cdot\left(-1\right)^3\)\(=-4\cdot\left(-27\right)+86\cdot3\cdot\left(-1\right)\)=108-258=-150Câu trả lời: a: \(P=7x\left(x^2y-5xy^2+2y^3\right)\)\(=7x\cdot x^2y-7x\cdot5xy^2+7x\cdot2y^3\)\(=7x^3y-35x^2y^2+14xy^3\)\(M=5y\left(x^2-3xy\right)\left(x-4y\right)\)\(=5y\left(x^3-4x^2y-3x^2y+12xy^2\right)\)\(=5y\cdot x^3-5y\cdot7x^2y+5y\cdot12xy^2\)\(=5x^3y-35x^2y^2+60xy^3\)\(N=\left(\dfrac{1}{2}x^4y^2-10x^2y^4\right):\left(-\dfrac{1}{4}xy\right)\)\(=-\dfrac{1}{2}x^4y^2:\dfrac{1}{4}xy+10x^2y^4:\dfrac{1}{4}xy\)\(=-2x^3y+40xy^3\)b: A=P-M-N\(=7x^3y-35x^2y^2+14xy^3-5x^3y+35x^2y^2-60xy^3+2x^3y-40xy^3\)\(=\left(7x^3y-5x^3y+2x^3y\right)+\left(-35x^2y^2+35x^2y^2\right)+\left(14xy^3-40xy^3-60xy^3\right)\)=\(4x^3y-86xy^3\)Thay x=-3;y=-1 vào A, ta được:\(A=4\cdot\left(-3\right)^3\cdot\left(-1\right)-86\cdot\left(-3\right)\cdot\left(-1\right)^3\)\(=-4\cdot\left(-27\right)+86\cdot3\cdot\left(-1\right)\)=108-258=-150