Thanks for your help, so for the gas problem I can just replace the D in the cost formula and then find the zero of the derivitive to find the min value. I'll try that. Also, I'm still kind of confused about the cylindrical can problem. I don't think that the radius and the height are related...
Alright so currently I'm learning about using zeros of first and second derivites to find relative max and mins in an equation. That stuff is pretty simply. Also we're learning to differentiate equations in relation to a variable that is not in the problem. But, I cannot figure out these...