Nebenbedingung
2x + 5y = 20 --> y = 4 - 0.4·x
Hauptbedingung
U = 0.5·LOG(x) + 0.5·LOG(y)
U = 0.5·(LOG(x) + LOG(y))
U = 0.5·LOG(x·y)
U = 0.5·LOG(x·(4 - 0.4·x))
U = 0.5·LOG(4·x - 0.4·x^2)
U = 0.5/LN(10)·LN(4·x - 0.4·x^2)
U' = 0.5/LN(10)·(4 - 0.8·x)/(4·x - 0.4·x^2) = 0
4 - 0.8·x = 0 --> x = 5