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\(A=-2+3\sqrt{x+1}\)

Vì \(3\sqrt{x+1}\ge0\forall x\Rightarrow A\ge-2\)

Dấu "=" xảy ra khi \(3\sqrt{x+1}=0\Leftrightarrow\sqrt{x+1}=0\Leftrightarrow x+1=0\Leftrightarrow x=-1\)

Vậy MinA = -2 <=> x = -1

Bài 2: 

a) Ta có: \(\left|2x-5\right|\ge0\forall x\)

\(\Leftrightarrow-\left|2x-5\right|\le0\forall x\)

\(\Leftrightarrow-\left|2x-5\right|+3\le3\forall x\)

Dấu '=' xảy ra khi \(x=\dfrac{5}{2}\)

Bổ sung điều kiện: \(x,y>0\)

\(A=\dfrac{x}{y}+\dfrac{y}{x}+\dfrac{xy}{x^2+y^2}\\ A=\dfrac{8}{9}\left(\dfrac{x}{y}+\dfrac{y}{x}\right)+\dfrac{1}{9}\left(\dfrac{x}{y}+\dfrac{y}{x}\right)+\dfrac{xy}{x^2+y^2}\\ A=\dfrac{8}{9}\left(\dfrac{x}{y}+\dfrac{y}{x}\right)+\left(\dfrac{x^2+y^2}{9xy}+\dfrac{xy}{x^2+y^2}\right)\)

Áp dụng BĐT cosi:

\(A\ge\dfrac{8}{9}\cdot2\sqrt{\dfrac{xy}{xy}}+2\sqrt{\dfrac{xy\left(x^2+y^2\right)}{9xy\left(x^2+y^2\right)}}=\dfrac{16}{9}+\dfrac{2}{3}=\dfrac{22}{9}\)

Vậy \(A_{min}=\dfrac{22}{9}\Leftrightarrow x=y\)

Z = { -2 ; -1 ; 0 ; 1 ; 2 ; 3 ; 4 ; 5 ; 6 ; 7 ; 8 ; 9}

\(A=\dfrac{5x^2}{x^2}-\dfrac{x}{x^2}+\dfrac{1}{x^2}=\dfrac{1}{x^2}-\dfrac{1}{x}+5=\left(\dfrac{1}{x^2}-\dfrac{1}{x}+\dfrac{1}{4}\right)+\dfrac{19}{4}=\left(\dfrac{1}{x}-\dfrac{1}{2}\right)^2+\dfrac{19}{4}\ge\dfrac{19}{4}\)

\(A_{min}=\dfrac{19}{4}\) khi \(\dfrac{1}{x}=\dfrac{1}{2}\Rightarrow x=2\)