Tìm x , y ,z thuộc Q biết: |x-3/4|+|2/5-y|+|x-y+z|=0

$\left|x-\frac{3}{4}\right|+\left|\frac{2}{5}-y\right|+\left|x-y+z\right|=0$ mà $\left|x-\frac{3}{4}\right|\ge 0;\left|\frac{2}{5}-y\right|\ge 0;\left|x-y+z\right|\ge 0$

$\Rightarrow \left\{\begin{array}{l}\left|x-\frac{3}{4}\right|=0\\ \left|\frac{2}{5}-y\right|=0\\ \left|x-y+z\right|=0\end{array}\right.\Rightarrow \left\{\begin{array}{l}x=\frac{3}{4}\\ y=\frac{2}{5}\\ z=y-x\end{array}\right.\Rightarrow \left\{\begin{array}{l}x=\frac{3}{4}\\ y=\frac{2}{5}\\ z=\frac{-7}{20}\end{array}\right.$ 

Vậy $x=\frac{3}{4}$; $y=\frac{2}{5}$; $z=-\frac{7}{20}$