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![](https://rs.olm.vn/images/avt/0.png?1311)
Ta có \(4x+4y+4z+4\sqrt{xyz}=16\Rightarrow4x+4\sqrt{xyz}+yz=yz-4y-4z+16\)
=> \(\left(2\sqrt{x}+\sqrt{yz}\right)^2=\left(4-y\right)\left(4-z\right)\Rightarrow\sqrt{\left(4-y\right)\left(4-z\right)}=2\sqrt{x}+\sqrt{yz}\)
=> \(\sqrt{x}\sqrt{\left(4-y\right)\left(4-z\right)}=\sqrt{x}\left(2\sqrt{x}+\sqrt{yz}\right)=2x+\sqrt{xyz}\)
Tương tự, rồi cộng lại, ta có
\(S=2\left(x+y+z\right)+3\sqrt{xyz}-\sqrt{xyz}=2\left(x+y+z+\sqrt{xyz}\right)=8\)
Vậy S=8
^_^
![](https://rs.olm.vn/images/avt/0.png?1311)
https://olm.vn/hoi-dap/tim-kiem?id=199649&subject=1&q=+++++++++++Cho+x,y,z%3E0.+Th%E1%BB%8Fa+m%C3%A3n:+x+y+z+%E2%88%9Axyz=4++T%C3%ADnh+Gi%C3%A1+tr%E1%BB%8B+c%E1%BB%A7a+bi%E1%BB%83u+th%E1%BB%A9c:A=%E2%88%9Ax(4%E2%88%92y)(4%E2%88%92z)+%E2%88%9Ay(4%E2%88%92z)(4%E2%88%92x)+%E2%88%9Az(4%E2%88%92x)(4%E2%88%92y)%E2%88%92%E2%88%9Axyz++++++++++
Bạn tự tham khảo nhé
![](https://rs.olm.vn/images/avt/0.png?1311)
Bạn ghi sai đề thì phải giả thiết phải là \(x+y+z+\sqrt{xyz}=4\)
Khi đó suy ra \(4\left(x+y+z\right)+4\sqrt{xyz}=16\)
Ta có: \(x\left(4-y\right)\left(4-z\right)=x[16-4\left(y+z\right)+yz]=x[4\left(x+y+z\right)+4\sqrt{xyz}-4\left(y+z\right)+yz]\)
\(=x\left(4x+4\sqrt{xyz}+yz\right)=x\left(2\sqrt{x}+\sqrt{yz}\right)^2\)
\(\Rightarrow\sqrt{x\left(4-y\right)\left(4-z\right)}=\sqrt{x}\left(2\sqrt{x}+\sqrt{yz}\right)=2x+\sqrt{xyz}\)
tương tự \(\left\{{}\begin{matrix}\sqrt{y\left(4-z\right)\left(4-x\right)}=2y+\sqrt{xyz}\\\sqrt{z\left(4-x\right)\left(4-y\right)}=2z+\sqrt{xyz}\end{matrix}\right.\)
Cộng lại ta được VT\(=\) \(2\left(x+y+z+\sqrt{xyz}\right)+\sqrt{xyz}\) \(=8+\sqrt{xyz}\)(điều phải chứng minh)
![](https://rs.olm.vn/images/avt/0.png?1311)
\(x+y+z+\sqrt{xyz}=4\)
\(\Leftrightarrow xyz=\left(4-x-y-z\right)^2\)
\(\Leftrightarrow xyz=16+x^2+y^2+z^2-8x-8y-8z+2xy+2xz+yz\)
\(\sqrt{x\left(4-y\right)\left(4-z\right)}=\sqrt{x\left(16-4y-4z+yz\right)}=\sqrt{16x-4xy-4xz+xyz}\)
\(=\sqrt{16x-4xy-4xz+16+x^2+y^2+z^2-8x-8y-8z+2xy+2yz+2xz}\)
\(=\sqrt{8x-2xy-2xz+2yz+x^2+y^2+z^2-8y-8z+16}\)
\(=\sqrt{\left(-x+y+z-4\right)^2}=\left|y+z-x-4\right|=\left|y+z-x-\left(x+y+z+\sqrt{xyz}\right)\right|\)
\(=\left|-2x-\sqrt{xyz}\right|=2x+\sqrt{xyz}\) (Vì x > 0)
Tương tự : \(\sqrt{y\left(4-z\right)\left(4-x\right)}=2y+\sqrt{xyz}\) , \(\sqrt{z\left(4-x\right)\left(4-y\right)}=2z+\sqrt{xyz}\)
Suy ra \(B=2x+2y+2z+2\sqrt{xyz}=2\left(x+y+z+\sqrt{xyz}\right)=2.4=8\)
![](https://rs.olm.vn/images/avt/0.png?1311)
Ta có \(x+y+z+\sqrt{xyz}=4\Rightarrow4x+4y+4z+4\sqrt{xyz}=16\)
Ta lại có \(\sqrt{x\left(4-y\right)\left(4-z\right)}=\sqrt{x\left(16-4y-4z+yz\right)}=\sqrt{x\left(4x+4\sqrt{xyz}+yz\right)}=\sqrt{4x^2+4x\sqrt{xyz}+xyz}=\sqrt{\left(2x+\sqrt{xyz}\right)^2}=2x+\sqrt{xyz}\)
Tương tự \(\sqrt{y\left(4-z\right)\left(4-x\right)}=2y+\sqrt{xyz}\)
\(\sqrt{z\left(4-x\right)\left(4-y\right)}=2z+\sqrt{xyz}\)
Suy ra \(P=\sqrt{x\left(4-y\right)\left(4-z\right)}+\sqrt{y\left(4-z\right)\left(4-x\right)}+\sqrt{z\left(4-x\right)\left(4-y\right)}-\sqrt{xyz}=2x+\sqrt{xyz}+2y+\sqrt{xyz}+2z+\sqrt{xyz}-\sqrt{xyz}=2x+2y+2z+2\sqrt{xyz}=2\left(x+y+z+\sqrt{xyz}\right)=2.4=8\)
tu gia thiet => \(4x+4y+4z+4\sqrt{xyz}=16\)
Xet \(\sqrt{x\left(4-y\right)\left(4-z\right)}=\sqrt{x\left(16-4y-4z+yz\right)}\)
= \(\sqrt{x\left(4x+4y+4y+4\sqrt{xyx}-4y-4z+yz\right)}\)
=\(\sqrt{x\left(4x+4\sqrt{xyz}+yz\right)}\)
=\(\sqrt{4x^2+4x\sqrt{xyx}+xyz}=\sqrt{\left(2x+\sqrt{xyz}\right)^2}\)
= \(2x+\sqrt{xyz}\)
tuong tu va suy ra \(\sqrt{x\left(4-y\right)\left(4-z\right)}+\sqrt{y\left(4-z\right)\left(4-x\right)}+\sqrt{z\left(4-x\right)\left(4-y\right)}\)
= \(2\left(x+y+z\right)+3\sqrt{xyz}\)
hinh nhu de bai bn viet thieu \(-\sqrt{xyz}\)
neu dung de thi goi bieu thuc can tinh la A
ta co \(A=2\left(x+y+z\right)+2\sqrt{xyz}=2\left(x+y+z+\sqrt{xyz}\right)=2.4=8\)
Chuc ban hoc tot
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