Calculus 3 Help needed-Minimizing

I'm self teaching myself multivariable calculus and have gotten stuck on problems involving minimizing and maximizing. The two problems that I am currently stuck on are as follows.

1)You are creating a retangular box with a volume of 32000cm^3. Find dimensions that minimize the material used in the construction of this box.

2) Find three numbers whose sum add to 100 and whose product is a minimum.

When I work both problems my answers come out as maximums and not minimums, assistance will be appreciated very much.

Thank you