If the length (front) of the ground is x meters and the width (side) is y meters, then xy=600 and y=600/x, then the front wall is 3x*800 yuan, the side wall is 2*3y*600 yuan, and the roof is 6000 yuan.

Total cost z = 2400x+3600y+6000xy = 2400x+2160000/x.

+6000 & gt； = 2 root number (2400*2 160000)+6000.

=144000+6000 =150000 yuan, the equal sign is true if and only if 2400x=2 160000/x, that is, x=30. It can be seen that the total cost is the lowest when the front wall is 30 meters long and the side wall is 20 meters long.

Note: (1) The above calculation is based on the roof cost of 6,000 yuan instead of 6,000 yuan per square meter.

(2) If you learn the derivative and find the objective function, you can also use the derivative to find the minimum value of the objective function.