Có $x^2-3x=y\Rightarrow y^2=(x^2-3x)^2\Rightarrow 6x^3-9x^2+y^2=x^4$$S=3x^3+y^2-8x^2+3x(x^2-3x)+1$$=x^4-8x^2+1$Có $x^2-3x=y\leq 0\Rightarrow x\in [0;3]\Rightarrow x^2\in [0;9]$ nên$S min=-15\Leftrightarrow x=2$$S max=10\Leftrightarrow x=3$
Có $x^2-3x=y\Rightarrow y^2=(x^2-3x)^2\Rightarrow 6x^3-9x^2+y^2=x^4$$S=3x^3+y^2-8x^2+3x(x^2-3x)+1=(6x^3-9x^2+y^2)-8x^2+1$$=x^4-8x^2+1$Có $x^2-3x=y\leq 0\Rightarrow x\in [0;3]\Rightarrow x^2\in [0;9]$ nên$S min=-15\Leftrightarrow x=2$$S max=10\Leftrightarrow x=3$