xy-12=4x+y≥2√(4xy)=4√(xy)
xy-4√(xy)-12≥0
(√(xy)-6)(√(xy)+2)≥0
√(xy)≤-2,√(xy)≥6
因为√(xy)≥0
所以√(xy)≥6
xy≥36
所以最小值=36