Question

31. Với x∈Z . So sánh x² và 3x

Answer 1

+)Ta xét 3 TH:

*TH1:x<3

\(\Rightarrow x^2=x.x< 3x\)

\(\Rightarrow x^2< 3x\)

*TH2:x=3

\(\Rightarrow x^2=x.x=3x\)

\(\Rightarrow x^2=3x\)

*TH3:x>3

\(\Rightarrow x^2=x.x>3.x\)

\(\Rightarrow x^2>3x\)

Chúc bn học tốt

Answer 2

TH1: \(x< 0\Leftrightarrow\left\{{}\begin{matrix}x^2>0\\3x< 0\end{matrix}\right.\)\(\Leftrightarrow x^2>3x\)

TH2: \(x=0\Leftrightarrow x^2=3x=0\)

TH3: \(x>0\):

+) \(0< x< 3\Leftrightarrow x^2< 3x\)

+) \(x=3\Leftrightarrow x^2=3x\)

+) \(x\ge4\Leftrightarrow x^2>3x\)

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