40X+90Y+20XY

不妨设xy≥0,40x+90y+20xy≤32000≥4x+9y+2xy-320≥12√(xy)+2xy-320=2xy+12√(xy)-320=2{[√(xy)]^2+6√(xy)+9}-338=2[√(xy)+3]^2-3382[√(xy)+3]^2-338≤0[√(xy)+3]^2-169≤0-13≤√(xy)+3≤13-16≤√(xy)≤100...