40X+90Y+20XY

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40X+90Y+20XY
40X+90Y+20XY

40X+90Y+20XY
不妨设xy≥0,
40x+90y+20xy≤3200
0≥4x+9y+2xy-320≥12√(xy)+2xy-320
=2xy+12√(xy)-320
=2{[√(xy)]^2+6√(xy)+9}-338
=2[√(xy)+3]^2-338
2[√(xy)+3]^2-338≤0
[√(xy)+3]^2-169≤0
-13≤√(xy)+3≤13
-16≤√(xy)≤10
0≤√(xy)≤10
xy≤100

用基本不等式.再解不等式

楼上用均值不等式解题，应注意x大于零，y大于零

40X+90Y+20XY 已知:x-xy=40,xy-y=-20,求代数式x-y和x+y-2xy的值. x-xy=40,xy-y= -20,求代数式x-y和x+y-2xy算试已知x—xy=40,xy—y=—20,求代数式x—y和x+y—2xy的值已知x-xy=40,xy-y=-20,求x-y和x+y-2xy的值.要有过程,把你怎么想的写出来. |x|/x+y/|y|+xy/|xy|= x^2+xy+x=36,y^2+xy+y=20,求x+y. xy+x=20 xy+y=18急 xy+xy=20,x=,y= 【x(xy-xy)-y(x-xy)】÷3xy 已知x-xy=20,xy-y=-10,则x-y=( ),x+y-2xy=( ). 已知x-xy＝20,xy-y＝-10,则x-y＝?x+y-2xy＝? 已知x-xy=20,xy-y=-10,则x-y=?,x+y-2xy? 已知x-xy=20,xy-y=12,则x-y= ,-2xy+x+y= 已知x-xy=20 xy-y=12,则x-y=（）,x+y-2xy=（）. 已知:x-xy=20,xy-y=12,则x-y=...,-2xy+x+y=... -x-3xy+y xy-1+x-y