\(\sqrt{2x-5}+2\sqrt{7-x}=\sqrt{3}x^2-8\sqrt{3}x+19\sqrt{3}\left(đk:\frac{5}{2}\le x\le7\right)\)(*)
Có \(\left(\sqrt{2x-5}+2\sqrt{7-x}\right)^2=\left(\sqrt{2x-5}+\sqrt{2}.\sqrt{14-2x}\right)^2\le\left(1+2\right)\left(2x-5+14-2x\right)\)(áp dụng bđt bunhiacopski)
<=> \(\left(\sqrt{2x-5}+2\sqrt{7-x}\right)^2\le3.9\)
=> \(\sqrt{2x-5}+2\sqrt{7-x}\le\sqrt{3.9}=3\sqrt{3}\) (1)(do \(\sqrt{2x-5}+2\sqrt{7-x}\ge0\))
Có \(\sqrt{3}x^2-8\sqrt{3}x+19\sqrt{3}=\sqrt{3}\left(x^2-8x+16\right)+3\sqrt{3}=\sqrt{3}\left(x-4\right)^4+3\sqrt{3}\ge3\sqrt{3}\)(2)
Từ (1),(2) => Dấu "=" xảy ra<=> \(\left\{{}\begin{matrix}\sqrt{14-2x}=\sqrt{2x-5}.\sqrt{2}\\x-4=0\end{matrix}\right.\) <=>\(\left\{{}\begin{matrix}14-2x=4x-10\\x=4\end{matrix}\right.\)
<=> \(\left\{{}\begin{matrix}x=4\\x=4\end{matrix}\right.\) => x=4(t/m)
Vậy pt (*) có tập nghiệm \(S=\left\{4\right\}\)
@Lê Thị Thục Hiền @Vũ Minh Tuấn
