\(a.A=4-x^2+2x\)
\(A=-\left(x^2-2x-4\right)\)
\(A=-\left(x^2-2x+1-5\right)\)
\(A=-\left(x-1\right)^2+5\)
Ta có: \(-\left(x-1\right)^2\le0\forall x\in R\)
\(\Rightarrow-\left(x-1\right)^2+5\le5\)
\(\Rightarrow Max_A=5\Leftrightarrow x=1\)
\(b.B=4x-x^2\)
\(B=-\left(x^2-4x\right)\)
\(B=-\left(x^2-4x+4-4\right)\)
\(B=-\left(x-2\right)^2+4\le4\forall x\in R\)
\(\Rightarrow Max_B=4\Leftrightarrow x=2\)
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