b: \(A=\left(2x-y\right)\left(4x^2+2xy+y^2\right)+\left(y-3x\right)\left(y^2+3xy+9y^2\right)\)
\(=\left(2x\right)^3-y^3+y^3-\left(3x\right)^3\)
\(=8x^3-27x^3=-19x^3\)
Khi \(x=-\dfrac{1}{2}\) thì \(A=-19\cdot\left(-\dfrac{1}{2}\right)^3=-19\cdot\dfrac{-1}{8}=\dfrac{19}{8}\)
c: \(x+y=1\)
=>\(\left(x+y\right)^2=1\)
=>\(x^2+y^2+2xy=1\)
=>2xy=1-5=-4
=>xy=-2
\(B=x^3+y^3=\left(x+y\right)^3-3xy\left(x+y\right)\)
\(=1^3-3\cdot\left(-2\right)\cdot1\)
=1+6=7
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