\(A=a^2\)
\(B=2a\)
\(C=A-B=a^2-2a=a\left(a-2\right)\)
(1) nếu \(\left[\begin{matrix}a=0\\a=2\end{matrix}\right.\)\(\Rightarrow C=0\Rightarrow A=B\)
(2) nếu \(a< 0\Rightarrow\left\{\begin{matrix}a< 0\\a-2< 0\end{matrix}\right.\)\(\Rightarrow C>0\Rightarrow A>B\)
(3) nếu \(0< a< 2\) \(\Rightarrow\left\{\begin{matrix}a>0\\a-2< 0\end{matrix}\right.\)\(\Rightarrow C< 0\Rightarrow A< B\)
(4) nếu a>2 \(\Rightarrow\left\{\begin{matrix}a>0\\a-2>0\end{matrix}\right.\)\(\Rightarrow C>0\Rightarrow A>B\)
Ta có: \(a^2=a.a\)
\(2a=a+a\)
\(\Rightarrow a.a>a+a\) (\(nhân>chia\))
hay \(a^2>2a\)
Vậy \(a^2>2a\).
