I'm practicing how to differentiate equations. I need help differentiation this: http://img127.imageshack.us/img127/531/voloz4.jpg [Broken] I made that equation up from a soda cup, different size bases. Is this equation correct? I tried to differentiate with respect to time. I get http://img112.imageshack.us/img112/9403/diffcb7.jpg [Broken]. Does that look right? Thanks for any help. Peter H.