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Các thánh giúp e nha Ace Legona Nguyễn Huy Tú Toshiro Kiyoshi Phương An Akai Haruma @Nguyễn Vũ Phượng Thảo
\(M=\dfrac{xy+2x+1}{xy+x+y+1}+\dfrac{yz+2y+1}{yz+y+z+1}+\dfrac{xz+2z+1}{xz+z+x+1}\)
\(M=\dfrac{xy+x+x+1}{x\left(y+1\right)+y+1}+\dfrac{yz+y+y+1}{y\left(z+1\right)+z+1}+\dfrac{xz+z+z+1}{z\left(x+1\right)+x+1}\)
\(\Rightarrow M=\dfrac{x\left(y+1\right)+x+1}{\left(x+1\right)\left(y+1\right)}+\dfrac{y\left(z+1\right)+y+1}{\left(y+1\right)\left(z+1\right)}+\dfrac{z\left(x+1\right)+z+1}{\left(z+1\right)\left(x+1\right)}\)
Quy đồng là xong nha
Do \(xyz\ne0\) ta có:
\(\dfrac{1}{xy}+\dfrac{1}{xz}+\dfrac{1}{yz}=0\Leftrightarrow xyz\left(\dfrac{1}{xy}+\dfrac{1}{xz}+\dfrac{1}{yz}\right)=0\Leftrightarrow x+y+z=0\)
Lại có: \(x^3+y^3+z^3=x^3+y^3+3x^2y+3y^2x-3xy\left(x+y\right)+z^3\)
\(=\left(x+y\right)^3+z^3-3xy\left(-z\right)=\left(x+y+z\right)\left(\left(x+y\right)^2-\left(x+y\right)z+z^2\right)+3xyz=3xyz\)
Vậy nếu \(x+y+z=0\) thì \(x^3+y^3+z^3=3xyz\)
\(P=\dfrac{x^2}{yz}+\dfrac{y^2}{xz}+\dfrac{z^2}{xy}=\dfrac{x^3}{xyz}+\dfrac{y^3}{xyz}+\dfrac{z^3}{xyz}=\dfrac{x^3+y^3+z^3}{xyz}=\dfrac{3xyz}{xyz}=3\)
Để M xác định thì \(x,y,z\ne0\)
\(xy+xz+yz=0\Rightarrow\left\{{}\begin{matrix}\dfrac{xy}{z}+x+y=0\\\dfrac{xz}{y}+x+z=0\\\dfrac{yz}{x}+y+z=0\end{matrix}\right.\)
Cộng vế với vế ta được:
\(\dfrac{xy}{z}+\dfrac{xz}{y}+\dfrac{yz}{x}+2\left(x+y+z\right)=0\)
\(\Leftrightarrow M+2.\left(-1\right)=0\Rightarrow M=2\)
Ta có :
\(xy+yz+xz=0\\ \Rightarrow\left[{}\begin{matrix}xy=-xz-yz=-z\left(x+y\right)\\yz=-xy-xz=-x\left(y+z\right)\\xz=-xy-yz=-y\left(x+z\right)\end{matrix}\right.\)
\(M=\dfrac{xy}{z}+\dfrac{xz}{y}+\dfrac{yz}{x}=\dfrac{-z\left(x+y\right)}{z}+\dfrac{-y\left(x+z\right)}{y}+\dfrac{-x\left(y+z\right)}{x}\\ =-\left(x+y\right)-\left(x+z\right)-\left(y+z\right)=-x-y-x-z-y-z\\ =-2\left(x+y+z\right)=\left(-2\right)\cdot\left(-1\right)=2\)
\(\Rightarrow M=2\)
Áp dụng công thức a3+b3+c3=3abc
Bài làm
Đặt \(\dfrac{1}{x}\)= a, \(\dfrac{1}{y}\)= b, \(\dfrac{1}{z}\)= c
vì a+b+c = 0 nên a3+b3+c3=3abc
S= \(\dfrac{yz}{x^2}\)+ \(\dfrac{xz}{y^2}\)+ \(\dfrac{xy}{z^{ }2}\)
=\(\dfrac{xyz}{x^{ }3}\)+\(\dfrac{xyz}{y^{ }3}\)+\(\dfrac{xyz}{z^{ }3}\) = xyz(\(\dfrac{1}{x^3}\)+\(\dfrac{1}{y^{ }3}\)+\(\dfrac{1}{z^{ }3}\))
= xyz ( a3+b3+c3 )
= xyz \(\times\)3abc = xyz \(\times\) \(\dfrac{3}{xyz}\) = 3
a)
\(\dfrac{x^2+x-6}{x^3-4x^2-18x+9}=\dfrac{x^2+3x-2x-6}{x^3+3x^2-7x^2-21x+3x+9}\)
\(=\dfrac{x\left(x+3\right)-2\left(x+3\right)}{x^2\left(x+3\right)-7x\left(x+3\right)+3\left(x+3\right)}\)
\(=\dfrac{\left(x-2\right)\left(x+3\right)}{\left(x^2-7x+3\right)\left(x+3\right)}=\dfrac{x-2}{x^2-7x+3}\)