Giải thích các bước giải:
3.Ta có:
$M=\dfrac{-3}{\sqrt{x}+3}$
$\to A=x\cdot \dfrac{-3}{\sqrt{x}+3}+\dfrac{4x+7}{\sqrt{x}+3}$
$\to A= \dfrac{-3x}{\sqrt{x}+3}+\dfrac{4x+7}{\sqrt{x}+3}$
$\to A= \dfrac{-3x+4x+7}{\sqrt{x}+3}$
$\to A= \dfrac{x+7}{\sqrt{x}+3}$
$\to A= \dfrac{x-9+16}{\sqrt{x}+3}$
$\to A= \dfrac{(\sqrt{x}+3)(\sqrt{x}-3)+16}{\sqrt{x}+3}$
$\to A=(\sqrt{x}-3)+\dfrac{16}{\sqrt{x}+3}$
$\to A=(\sqrt{x}+3)+\dfrac{16}{\sqrt{x}+3}-6$
$\to A\ge 2\sqrt{(\sqrt{x}+3)\cdot\dfrac{16}{\sqrt{x}+3}}-6$
$\to A\ge 2$
Dấu = xảy ra khi $(\sqrt{x}+3)=\dfrac{16}{\sqrt{x}+3}\to x=1$
