If x is negative, is x 9 (2) x3

Question

If x is negative, is x 9
(2) x3 < -9

celiaXDN · Answer

X^2 and X^3?

If it is the case, then the answer should be E.
1) X > 3 or X<-3

2) when x= -2.5, X^3 <-9.

giddi77 · Answer

Is it B? 1) x^2 < 9 => x < -3 or or x > 3 .INSUFF. 2) x^3 < -9 => x < -3. Definitely true since x > -3 say x = -2 => x^3 > -9

chiragr · Answer

x is negative

x^2 > 9 => +-x > 3 => x > 3 which is not the case as x is negative... second -x > 3 => x <-3 which is sufficient to answer quesiton...

x^3 < -9 x is not integer so -2^3 is -8 and -2.5^3 < -9 so insufficient

So A is only sufficient!

ps_dahiya · Answer

A Main stem x<0 St1: x^2 > 9 means either x<-3 or x> 3. But x can not be +ve as per main stem. So SUFF. St2: x^3< -9 means x<-2.1. So INSUFF

celiaXDN · Answer

Yeah, it should be A

I overlook the part that x is negative.

chiragr · Answer

Well ineuqlities are always a beast for me.... let me expalain you why, still sometime, I get confused !

lets say x^2 > y^2 =>

+-x > +- y

This gives four possibilities!
1) x > y
2) -x> y

3) x> -y => -x<y
4) -x>-y => x<y

now see 1 and 4 are exactly opposite of each other and so do 2, 3

Now am I mathematically wrong if not than

x^2 > 9 means what !

Any insight with proof!

ywilfred · Answer

(1) x = 3 or x = -3. So we know x is definitely not less than -3. Sufficient.

(2) x^3 < -9 -> works out to about between -2 and -3. Again, we know it's not less than -3. Sufficient.

Ans D

chiragr · Answer

I will create new thread for inequality so everyone answers!

celiaXDN · Answer

For 2) 

-2.5^3 <-9 (-2.5>-3)

-4^3 <-9 (-4<-3)

instuff.

If x is negative, is x < -3? (1) x2 > 9 (2) x3 < -9

Prep Toolkit

Top 5 GMAT Debrief Videos

615 to 715 in 15 Days (Ria's Strategies)

More than Quant/Verbal Abilities: Speed vs. Accuracy

745 with Only Self-Prep and GMAT Club

From 555 to 765: 210-point Improvement

Perfect 805 Score Debrief by Julia

My Rewards