A thin-walled rectangular tube has uniform thickness t and dimensions a ( b to the median line

Question:

A thin-walled rectangular tube has uniform thickness t and dimensions a ( b to the median line of the cross section (see figure).
How does the shear stress in the tube vary with the ratio ( = a/b if the total length Lm of the median line of the cross section and the torque T remain constant?
From your results, show that the shear stress is smallest when the tube is square (( = 1).