Optimizing Wetted Area in a Rotating Disk Evaporator

[momu]

A circular disk of radius r is used in an evaporator and is rotated in a vertical plane. It is to be partially submerged in the liquid so as to maximize the exposed wetted area of the disk. show that the center of the disk should be positioned at height r/sqrt(1+pi^2)

ok i absolutely know how to do this question but I'm having a tough time deriving the equation for the submerged portion.. if i can get help on that i would be greatful thanks.

 

[momu]

Nevermind haha