Câu hỏi của Võ Tuấn Nguyên

3.\(A=x^3-3x^2y+3xy^2-y^3+3x^2y-3xy^2-\left(x^2-2xy+y^2\right)\)\(=\left(x-y\right)^3+3xy\left(x-y\right)-\left(x-y\right)^2\)\(=\left(-5\right)^3+3.\left(-6\right).\left(-5\right)-\left(-5\right)^2\)\(=-60\)4.\(B=-\left(x^2-6x+9\right)+21=-\left(x-3\right)^2+21\)Do \(-\left(x-3\right)^2\le0;\forall x\)\(\Rightarrow B\le21\)\(B_{max}=21\) khi \(x=3\)Câu trả lời: 3.\(A=x^3-3x^2y+3xy^2-y^3+3x^2y-3xy^2-\left(x^2-2xy+y^2\right)\)\(=\left(x-y\right)^3+3xy\left(x-y\right)-\left(x-y\right)^2\)\(=\left(-5\right)^3+3.\left(-6\right).\left(-5\right)-\left(-5\right)^2\)\(=-60\)4.\(B=-\left(x^2-6x+9\right)+21=-\left(x-3\right)^2+21\)Do \(-\left(x-3\right)^2\le0;\forall x\)\(\Rightarrow B\le21\)\(B_{max}=21\) khi \(x=3\)