1)
2X+3Y=120
Y=40-2X/3
U=XY=X(40-2X/3)
=-2X^2/3+40X
=-2/3(X-30)^2+600
X=30,Y=20
U=600
2)
U=(X+1)(Y+1)-XY
=X+Y+1
U=XY
X=0,Y=0
U=0
0<=U<=600
3)他的收入应该是K
U=XY
2X+3Y=120
3Y=120-2X
U=XY=X(40-2X/3)
=-2X^2/3+40X
2*1.44X+3Y=K
3Y=K-2.88X
U=XY=X(K-2.88X/3)
=XK-2.88X^2/3=-2X^2/3+40X
XK=0.88X^2/3+40X
K=0.88X/3+40(0<=X<=60)
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