Front area: 12/x*3=36/x
Front-end cost: 400*36/x= 14400/x
Side area: 3*x*2=6x
Side cost: 150*6x=900x.
House price: y = 14400/x+900x+5800 (0
(2) y'=- 14400/x? +900
Single increment interval: y'>0
- 14400/x? +900 & gt; 0
x? & gt 14400/900
X & lt-4 or x> IV
Single subtraction interval: -4
When y'=0, y gets the minimum value.
x? = 14400/900
x= 4
When x=4, y takes the minimum value: y =14400/4+900 * 4+5800 = 23800 yuan.