数学Jj 274

a8a9 · 发表于 2013-7-28 22:33:31

请教274题如果知道xy＜0 and xz＜0, 就说明z和y都必须在x和0的右边，所以应该是c而不是e啊。大家怎么看?

jolhui · 发表于 2013-7-28 22:55:40

有道理，求解！顺便问下其他题还残的严重吗。。。

bluejake · 发表于 2013-7-29 09:40:04

俺觉的还是E吧要比xyz与x的大小也就说x(yz-1)是否大于0 但怎么画图找也找不到yz与1的关系---虽然不是很清晰但是感觉还是选E 年纪大了脑子乱

a8a9 · 发表于 2013-7-30 07:01:02

actually, you're right. it should be E. my reasoning is that xy < 0 and x is on the left side of the y, then x must be less than 0 and y must be greater than 0; same apply to xz<0. Therefore, with x is negative, y and z are positive, the product of xyz will be negative but we don't know whether it is left side of x or right side of x. Example: x=-1, y=1, z=1 => can't be determined. x=-1, y = 2, z=2 ==>left side of x.

数学Jj 274

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