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[解法]
<Sol.>
4x2+9y2-20x+24y+42
=[(2x)2-2(2x)(5)+52]+[(3y)2+2(3y)(4)+42]+(42-25-16)
=(2x-5)2+(3y+4)2+1
[3(2x-5)-4(3y+4)]2£[(2x-5)2+(3y+4)2][32+(-4)2]
(6x-12y-31)2£[(2x-5)2+(3y+4)2]´25
(10)2£25[(2x-5)2+(3y+4)2]
(2x-5)2+(3y+4)2³4
(2x-5)2+(3y+4)2+1³5
-8x+20=9y+12
得8x+9y=8
又(x, y)滿足6x-12y=41
亦即(x, y)滿足
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