Derivative, some difficulties.

Can someone please help me to solve this two problems? I've been sitting for hours trying to understand them.

Here it goes.

A 30 cm string was cut in two pieces. One string was formed as a circle and the second string was shaped as a quadrat.

Show that the sum of the circle and the quadrat's area always exceed 30 cm² regardless where the cut is made.

__Found a third problem:__

A factory are going to to make cylindershaped cans. The sum of the diameter and height are 80 cm. which measures gives the greatest volume?

Please help me with this!

/Attack.