A piece of wire of length $20$cm is cut into $2$ parts. the first part is bent into a circle of radius $r$ in cm, the second into a square of side length $s$ in cm.
a) write down an expression for the sum of the perimeters of the two shapes in terms of r and s. use this to express $s$ in terms of $r$
I have got $2πr+4s=20$ but don't even know if this is right or not
b) find an expression for $S$, the sum of the areas enclosed by the two shapes in terms of r
c) use differentiation to determine the value of $r$ for which $S$ is a minimum
Really struggling with this as all other examples ask for minimum and maximum areas. I can't even figure out where to start so would appreciate any help!