Answer:
Combined cost C(x) = cx + 4c/(x+1)
minimum C(x) ==> C '(x) = 0
==> c - 4c/(x+1)2 = 0
==> 4c/(x+1)2 = c
==> (x+1)2 = 4
==> x +1 = 2
==> x = 1 mile
Hence she should live 1 mile from work to minimise comined costs.
Step-by-step explanation: