4x+y+12≥2√(4xy)+12
2√(4xy)+12≤xy
4√(xy)+12≤xy
xy-4√(xy)-12≥0
设 t=√(xy)
则,上式化为
t^2-4t-12≥0
(t-6)(t+2)≥0
所以 t≥6
所以 xy≥36
（xy)min=36