Giúp mình nhanh nhé ai nhanh cho ctlhn Cho 3 số x,y,z thỏa mãn : $\frac{1}{x}$ + $\frac{1}{y}$ + $\frac{1}{z}$ = $\frac{1}{x+y+z}$ Chứng minh rằng : $\frac{1}{x^{2019} }$ + $\frac{1}{y^{2019} }$ + $\frac{1}{z^{2019} }$ = $\frac{1}{x^{2019} +y^{2019} +z^{2019} }$

What does the doctor recommend to eat?

What do happy people do? 

What kind of food will help people to keep fit and stay healthy?