Cho $\frac{x}{y}$=$\frac{y}{z}$=$\frac{z}{t}$ Chứng minh $\frac{x^3+y^3+z^3}{y^3+z^3+t^3}$

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?