4xy-x-2y=4 4xy-(x+2y)=4 x+2y≧2√(2xy) 4xy-2√(2xy)-4≧0 令√(2xy)=t 则xy=t^2/2 2t^2-2t-4≧0 t^2-t-2≧0 (t-1/2)^2-9/4≧0 t-1/2≧3/2 (t>0) t≧2 √(2xy)=t≧2 xy的最小值是2.