This is university entrance exam in Vietnam.
A sector: economy, finance, bank, technology, science... university.
B sector: pharmacy, medicine university.
D sector: economy, foreign languages, society university.
May i ask a question ?
If the function of variables $\displaystyle x,y,z$ is convertable , ie $\displaystyle f(x,y,z) = f(x,z,y) = f(y,x,z) = f(y,z,x) =f(z,x,y) = f(z,y,x) $ , is there a theorem which states that $\displaystyle f(x,y,z) $ is min. or max when $\displaystyle x = y= z $ ???