math jj , pls

joywzy

1）DS. Is x<0? a) x^3 + x < 0 ==> x(x^2 +1) < 0 == > x , 0. Everyone agree A is right b) x^3 + x + 1 = 0 ==> x(x^2 + 1) = -1 == > x < 0 but someone said there is no real number resolution for B. so answer is A. D or A? do DS question really cares about exact answers? 【讨论】我认为是A，因为B方程无解。请其他机委提供看法

oddyuibe

I think the answer is D 1) x^3+x<0;;;x*(x^2+1)<0 since x^2+1>0, so x<0 2) x^3+x+1=0;;;x*(x^2+1)=-1 since x^2+1>0,so x<0 Thus, in my point of view, the answer should be D

oddyuibe

Personally,I think in DS,what we have to approve is whether x is in this range,while not a specific answer. Anyway, I think there should be a definite answer to condition 2, just beyond our capacity to work it out.

qiqi

我认为老几经正确应该是B
因为Y=X^3+X+1的曲线是有规律的，在实数范围绝对无解，由曲线可完全判断出。

joywzy

A, D都有道理.可到底是哪个?请高人确认一下.谢谢.

chanelcoco

d

likerock

I think it's "d" too. From b) , we can conclude that x = -1/(x^2+1). Since X^2 + 1 must be > 0, so x<0.