Inequalities trick

In addition to my previous post, I'd like to understand:

If the inequality is:
f(x) = (x+a)(x-b)^2(x+c)(x-d)^3 < 0
We are still going to work with the same number line from left to right starting with "+" and factors a,b,c,d placed on it in ascending order?
Since the number of factors remain an even number (4 total). Am I on the right track?

Yes, you can look at it like that too. You should always have the positive sign at the extreme right region. Do remember though that the factors must be of the form (x - a), (x - b) etc and not of the form (a - x).

Can somebody please explain for me why this one doesnt work that way?

Which of the following is a value of x for which x^11 - x^9 > 0 ?

A. -2
B. -1
C. -1/2
D. 1/2
E. 1


x11−x9>0 --> x9(x2−1)>0 --> (x+1)x9(x−1)>0 --> roots are -1, 0 and 1 --> −1<x<0 or x>1. Only C fits.

Answer: C.

But it does. Everything you have done is correct (ignoring the formatting issues).

I knew the answer makes sense, but if I apply the inequalities trick into this, with 3 roots -1, 0, and 1 and for the f(x) >0, shouldn't it be x <-1 and 0 <x<1?
Thanks

Dear All,

Is x a negative number?
(1) 9x > 10x
(2) is positive.

GMAT OG claims A is sufficient. But, what if x is a fraction number? Such as 2/90. In this case 2/90 also satisfies ST 1 ..
What is the exact solution for this?

This question is discussed here: is-x-a-negative-number-136882.html

Hope it helps.

if (k – 5)(k – 1)(k – 6) < 0
or
(k – 5)(k – 1)(k – 6) > 0
what will be range for both? I have tried method suggested by gurpreetsingh, but got confused when to take ">" and "<".
can anyone explain? It will be more helpful if anyone can upload an image of the solution.

Thanks.

Please explain how will it be different when you have <= or >=

Dear Ma'am!
Please try to respond to my pm...
really needed that help, if possible then revert on forum so that others can benefit for it too.
thanks

Karishma,
Pls tell me it is always be that signs will change like +-+- or it is possible the case when it will be the same repeats. for example, ++-+ or +--+

thank you

Thank you for your answer.
So I can put plus in the right segment and then just put -+-+ without trying numbers,arent I ?

Thanks a lot for this post , it's very helpful. Would be great if you clarify the below query:

Given:
If f(x) has three factors then the graph will have - + - +
If f(x) has four factors then the graph will have + - + - +

Question: What if we have 2 factors? do we need to start out interpretation as + - + and for 5 factors, do we need to have it in this way? - + - + - +

Thanks in advance,
Uma

Hi karishma, abhimahna, mikemcgarry, Bunuel, Skywalker18, and other experts

I am not able to understand the graph. How do we know which one is negative/positive? How does this graph help us in solving inequalities?

Thanks

Hi Shiv2016 ,

The concept is very easy.

1. Find out the zero points.
2. Arrange them in ascending order.
3. Draw them on a number line.
4. Take the right most as +ve and proceed towards left taking alternate signs.
5. If the inequality is of > form, then take all +ve ranges.
6. if the inequality is of lesser form, then take all -ve ranges.

E.g.

(x-2) (x-3 ) > 0

Equality form is greater than(>).

Zero points = 2 and 3

Draw them on number line. You will get 3 ranges. x<2; 2<x<3; and x >2.

Here, right most will be +ve, or x>2 will be +ve.
then 2<x<3 will be -ve
then x<2 will be +ve.

Since inequality is of (>) form, we will take all the ranges which have +ve sign.

Hence, the answer will be x>2 and x<2.

I hope it makes sense.

Dear Shiv2016,

I'm happy to respond.

I see that abhimahna already gave I wonderful explanation. I will just add a few points.

The post at the beginning of the thread is not 100% accurate and it is way to general for the GMAT. Inequalities with powers of x is a exceeding rare topic on the GMAT, and when there is a power in an inequality, it almost always is just x. I think you could take 50 GMATs in a row and not see a power of x higher than 2 in an inequality.

You may find this blog helpful:
GMAT Quadratic Inequalities

I definitely agree that finding the roots, the x-values that make the function zero, is the first step. Find these, and put these on a number line.

If the highest power is simply 2, then you should know that graph is a parabola. If the coefficient of \(x^2\) is positive, then the parabola opens upward, like the letter U. If the coefficient of \(x^2\) is negative, then the parabola opens downward, like an upside-down U. Right there, that should tell you where the function is positive or negative.

Higher powers of x (\(x^3\), \(x^4\), etc.) appear infrequently. The alternating pattern doesn't always work: in particular, the signs don't alternate if there's a repeated root, e.g. \(x^3 - 4x^2 + 4x > 0\)). That's a caveat for higher mathematics, but I don't think anyone would ever need to know this for the GMAT.

My friend, part of the reason you were confused is because the person who opened the thread was talking about more advanced math than you need for the GMAT. People who study engineering learn way more math than is needed for the GMAT, and some of them like to show off, but this doesn't really help anyone.

Does all this make sense?
Mike

[quote="gurpreetsingh"]I learnt this trick while I was in school and yesterday while solving one question I recalled.
Its good if you guys use it 1-2 times to get used to it.

Suppose you have the inequality

\(f(x) = (x-a)(x-b)(x-c)(x-d) < 0\)

Just arrange them in order as shown in the picture and draw curve starting from + from right.

now if f(x) < 0 consider curve having "-" inside and if f(x) > 0 consider curve having "+" and combined solution will be the final solution. I m sure I have recalled it fully but if you guys find any issue on that do let me know, this is very helpful.

Don't forget to arrange then in ascending order from left to right. \(a<b<c<d\)

So for f(x) < 0 consider "-" curves and the ans is: \((a < x < b)\), \((c < x < d)\)
and for f(x) > 0 consider "+" curves and the ans is: \((x < a)\), \((b < x < c)\), \((d < x)\)

If f(x) has three factors then the graph will have - + - +
If f(x) has four factors then the graph will have + - + - +

Hi, if the function is as such (-x-a)(-x-b), would we just flip the direction of the curve, and hence the order of signs? Thanks!