若x,y是有理数,且N=3x²+2y²-18x+8y+35,则N为正,为负?
人气:367 ℃ 时间:2019-09-10 08:55:47
解答
N=3(x-3)2+2(y+2)2
当且仅当x=3,y=-2时N=0,其余均为正
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