M=x+yxy.1z≥2√xyxy.1z=2z√xy≥2z(x+y2)=4z(x+y)M=x+yxy.1z≥2xyxy.1z=2zxy≥2z(x+y2)=4z(x+y)

=4z(1−z)=414−(z−12)2≥16=4z(1−z)=414−(z−12)2≥16

Min M= 16 khi  z=1/2 và  x=y =1/4

ĐKXĐ: \(\left\{{}\begin{matrix}2020-y^2\ge0\\2020-z^2\ge0\\2020-x^2\ge0\end{matrix}\right.\)

Ta có:

\(x\sqrt{2020-y^2}+y\sqrt{2020-z^2}+z\sqrt{2020-x^2}=3030\)

\(\Leftrightarrow2x\sqrt{2020-y^2}+2y\sqrt{2020-z^2}+2z\sqrt{2020-x^2}=6060\)

\(\Leftrightarrow2020-y^2-2x\sqrt{2020-y^2}+x^2+2020-z^2-2y\sqrt{2020-z^2}+y^2+2020-x^2-2z\sqrt{2020-x^2}+z^2=0\)

   \(\Leftrightarrow\left(\sqrt{2020-y^2}-x\right)^2+\left(\sqrt{2020-z^2}-y\right)^2+\left(\sqrt{2020-x^2}-z\right)^2=0\)

\(\Leftrightarrow\left(\sqrt{2020-y^2}-x\right)^2=\left(\sqrt{2020-z^2}-y\right)^2=\left(\sqrt{2020-x^2}-z\right)^2=0\)

\(\Leftrightarrow\left\{{}\begin{matrix}\sqrt{2020-y^2}=x\\\sqrt{2020-z^2}=y\\\sqrt{2020-x^2}=z\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}2020-y^2=x^2\\2020-z^2=y^2\\2020-x^2=z^2\end{matrix}\right.\)(vì \(x,y,z>0\))

\(\Leftrightarrow\left\{{}\begin{matrix}2020=x^2+y^2\\2020=y^2+z^2\\2020=z^2+x^2\end{matrix}\right.\)

\(\Rightarrow2\left(x^2+y^2+z^2\right)=3.2020\)

\(\Rightarrow x^2+y^2+z^2=3.1010=3030\)

\(\Rightarrow A=x^2+y^2+z^2=3030\)

Vậy \(A=3030\)

hay wa 😍

Lời giải:
Áp dụng BĐT AM-GM:

\(x\sqrt{2020-y^2}+y\sqrt{2020-z^2}+z\sqrt{2020-x^2}\leq \frac{x^2+(2020-y^2)}{2}+\frac{y^2+(2020-z^2)}{2}+\frac{z^2+(2020-x^2)}{2}=3030\)Dấu "=" xảy ra khi:

\(\left\{\begin{matrix} x^2=2020-y^2\\ y^2=2020-z^2\\ z^2=2020-x^2\end{matrix}\right.\Rightarrow x=y=z=\sqrt{1010}\)

Khi đó:

$A=3(\sqrt{1010})^2=3030$

https://diendantoanhoc.net/topic/182493-%C4%91%E1%BB%81-thi-tuy%E1%BB%83n-sinh-v%C3%A0o-l%E1%BB%9Bp-10-%C4%91hsp-h%C3%A0-n%E1%BB%99i-n%C4%83m-2018-v%C3%B2ng-2/

bài này năm trrong đề thi tuyển sinh vào lớp 10 ĐHSP Hà Nội Năm 2018 (vòng 2) bn có thể tìm đáp án trên mạng để tham khảo

\(\frac{x^2}{y+z}+\frac{y^2}{z+x}+\frac{z^2}{x+y}\ge\frac{\left(x+y+x\right)^2}{y+z+z+x+x+y}=\frac{x+y+x}{2}=1\)

Dấu ' =' xảy ra khi \(x=y=z=\frac{2}{3}\)

Pt đầu tương đương: \(\sqrt[3]{x^2}+2\sqrt[3]{y^2}+4\sqrt[3]{z^2}=7\)

Pt 2 tương đương:

\(\left(xy^2+z^4\right)^2-\left(xy^2-z^4\right)^2=4\)

\(\Leftrightarrow4xy^2z^4=4\)

\(\Leftrightarrow xy^2z^4=1\) (1)

Quay lại pt đầu, áp dụng AM-GM:

\(7=\sqrt[3]{x^2}+\sqrt[3]{y^2}+\sqrt[3]{y^2}+\sqrt[3]{z^2}+\sqrt[3]{z^2}+\sqrt[3]{z^2}+\sqrt[3]{z}\ge7\sqrt[7]{\sqrt[3]{x^2}.\sqrt[3]{y^4}.\sqrt[3]{z^8}}\)

\(\Leftrightarrow\sqrt[21]{x^2y^4z^8}\le1\)

\(\Leftrightarrow x^2y^4z^8\le1\)

\(\Rightarrow\left|xy^2z^4\right|\le1\Rightarrow xy^2z^4\le1\)

Dấu "=" xảy ra khi và chỉ khi:

\(\left\{{}\begin{matrix}x^2=y^2=z^2\\xy^2z^4=1\\x>0\end{matrix}\right.\) \(\Rightarrow\left\{{}\begin{matrix}x=1\\y=\pm1\\z=\pm1\end{matrix}\right.\)

Các bộ thỏa mãn là: \(\left(1;1;1\right);\left(1;1;-1\right);\left(1;-1;1\right);\left(1;-1;-1\right)\)

Anh ơi! Điều kiện x>0 là như nào ạ anh.