Question

So sánh: a² và 2a

Mình cần gấp lắm

Answer 1

\(A=a^2\)

\(B=2a\)

\(C=A-B=a^2-2a=a\left(a-2\right)\)

(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 \(\Rightarrow\left\{\begin{matrix}a>0\\a-2>0\end{matrix}\right.\)\(\Rightarrow C>0\Rightarrow A>B\)

Answer 2

Ta có: \(a^2=a.a\)

\(2a=a+a\)

\(\Rightarrow a.a>a+a\) (\(nhân>chia\))

hay \(a^2>2a\)

Vậy \(a^2>2a\).