\(a,B=\left(-\dfrac{1}{2}xy^3\right)\left(2x^3y\right)^2\)
\(=\left(-\dfrac{1}{2}.4\right)\left(x.x^6\right)\left(y^3.y^2\right)\)
\(=-2x^7y^6\)
\(b,x=2,y=\dfrac{1}{2}\Rightarrow B=-2.2^7.\dfrac{1}{2}^6=-4\)
\(a,B=\left(-\dfrac{1}{2}xy^3\right).\left(2x^3y\right)^2\\ =\left(-\dfrac{1}{2}\right).4.x.x^6.y^3.y^2\\ =-2x^7y^5\)
b, Thay \(x=2;y=\dfrac{1}{2}\) vào B
\(B=\left(-2\right).2^7.\left(\dfrac{1}{2}\right)^5=-8\)