Is A positive? x^2-2x+A is positive for all x Ax^2+1 is : GMAT Data Sufficiency (DS)
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# Is A positive? x^2-2x+A is positive for all x Ax^2+1 is

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Is A positive? x^2-2x+A is positive for all x Ax^2+1 is [#permalink]

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15 Jul 2010, 11:42
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Is A positive?

(1) x^2-2x+A is positive for all x
(2) Ax^2+1 is positive for all x
[Reveal] Spoiler: OA

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Last edited by Bunuel on 10 Apr 2012, 23:06, edited 1 time in total.
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15 Jul 2010, 13:40
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noboru wrote:
Is A positive?

x^2-2x+A is positive for all x
Ax^2+1 is positive for all x

OA is A

Is $$A>0$$?

(1) $$x^2-2x+A$$ is positive for all $$x$$:

Quadratic expression $$x^2-2x+A$$ is a function of of upward parabola (it's upward as coefficient of $$x^2$$ is positive). We are told that this expression is positive for all $$x$$ --> $$x^2-2x+A>0$$, which means that this parabola is "above" X-axis OR in other words parabola has no intersections with X-axis OR equation $$x^2-2x+A=0$$ has no real roots.

Quadratic equation to has no real roots discriminant must be negative --> $$D=2^2-4A=4-4A<0$$ --> $$1-A<0$$ --> $$A>1$$.

Sufficient.

(2) $$Ax^2+1$$ is positive for all $$x$$:

$$Ax^2+1>0$$ --> when $$A\geq0$$ this expression is positive for all $$x$$. So $$A$$ can be zero too.

Not sufficient.

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16 Jul 2010, 04:21
Hi,

I dont get it sorry... I mean I understand your equations Bunuel, but I tried first with picking numbers:

If I pick -0.5 for x --> x^2-2x+A>0 will hold for A > -1.25

...

Where is my mistake??
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16 Jul 2010, 07:01
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AndreG wrote:
Hi,

I dont get it sorry... I mean I understand your equations Bunuel, but I tried first with picking numbers:

If I pick -0.5 for x --> x^2-2x+A>0 will hold for A > -1.25

...

Where is my mistake??

The point here is that $$x^2-2x+A>0$$ for all $$x-es$$.

Let's do this in another way:

We have $$(x^2-2x)+A>0$$ for all $$x-es$$. The sum of 2 quantities ($$x^2-2x$$ and $$A$$) is positive for all $$x-es$$. So for the least value of $$x^2-2x$$, $$A$$ must make the whole expression positive.

So what is the least value of $$x^2-2x$$? The least value of quadratic expression $$ax^2+bx+c$$ is when $$x=-\frac{b}{2a}$$, so in our case the least value of $$x^2-2x$$ is when $$x=-\frac{-2}{2}=1$$ --> $$x^2-2x=-1$$ --> $$-1+A>0$$ --> $$A>1$$.

OR:

$$x^2-2x+A>0$$ --> $$x^2-2x+1+A-1>0$$ --> $$(x-1)^2+A-1>0$$ --> least value of $$(x-1)^2$$ is zero thus $$A-1$$ must be positive ($$0+A-1>0$$)--> $$A-1>0$$ --> $$A>1$$.

Hope it's clear.
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16 Jul 2010, 07:26
Wow u rock man!
That was very clear!

I especially like the +1 -1 trick

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16 Jul 2010, 08:36
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Hi Bunuel,

I really liked approached here but I still have some confusion,

Say for e.g if try to pick the numbers say x = -3

Then the equation in the first statement becomes

$$x^2 - 2x + A = 9 +6 +A = 15 + A >0$$

So now if we see A can have -ve and +ve values, isnt it ?

I am confused with this.

Please explain, whats wrong with this one.

Cheers
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16 Jul 2010, 08:49
nitishmahajan wrote:
Hi Bunuel,

I really liked approached here but I still have some confusion,

Say for e.g if try to pick the numbers say x = -3

Then the equation in the first statement becomes

$$x^2 - 2x + A = 9 +6 +A = 15 + A >0$$

So now if we see A can have -ve and +ve values, isnt it ?

I am confused with this.

Please explain, whats wrong with this one.

Cheers

Not every question can be solved by number picking.

For all $$x-es$$ means that no matter what $$x$$ you pick $$x^2 - 2x + A$$ must be positive. So it must be positive even for the lowest value of $$x^2 - 2x$$ which is -1 --> so $$-1+A$$ must be positive hence A must be more than 1.

Now again: if A>1 then for any $$x$$ expression $$x^2 - 2x + A$$ is positive.

But if A=-15 (or any other number less than 1) we can find some $$x-es$$ for which expression $$x^2 - 2x + A$$ is not positive, so theese values of A (values of $$A\leq{1}$$) are not valid.

Hope it's clear.
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16 Jul 2010, 08:56
Bunuel wrote:
nitishmahajan wrote:
Hi Bunuel,

I really liked approached here but I still have some confusion,

Say for e.g if try to pick the numbers say x = -3

Then the equation in the first statement becomes

$$x^2 - 2x + A = 9 +6 +A = 15 + A >0$$

So now if we see A can have -ve and +ve values, isnt it ?

I am confused with this.

Please explain, whats wrong with this one.

Cheers

Not every question can be solved by number picking.

For all $$x-es$$ means that no matter what $$x$$ you pick $$x^2 - 2x + A$$ must be positive. So it must be positive even for the lowest value of $$x^2 - 2x$$ which is -1 --> so $$-1+A$$ must be positive hence A must be more than 1.

Now again: if A>1 then for any $$x$$ expression $$x^2 - 2x + A$$ is positive.

But if A=-15 (or any other number less than 1) we can find some $$x-es$$ for which expression $$x^2 - 2x + A$$ is not positive, so theese values of A (values of $$A\leq{1}$$) are not valid.

Hope it's clear.

I understood the approach but the fact which is baffling me is that say the equation after subsituting value of x=-3 i.e 15+ A > 0 now we can have a value of A=-3 or may be -4 etc and still have the value of the equation in statement 1 as +ve

Am I thinking too much or just lacking some thing basic concept.

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16 Jul 2010, 09:16
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nitishmahajan wrote:
Bunuel wrote:
nitishmahajan wrote:
Hi Bunuel,

I really liked approached here but I still have some confusion,

Say for e.g if try to pick the numbers say x = -3

Then the equation in the first statement becomes

$$x^2 - 2x + A = 9 +6 +A = 15 + A >0$$

So now if we see A can have -ve and +ve values, isnt it ?

I am confused with this.

Please explain, whats wrong with this one.

Cheers

Not every question can be solved by number picking.

For all $$x-es$$ means that no matter what $$x$$ you pick $$x^2 - 2x + A$$ must be positive. So it must be positive even for the lowest value of $$x^2 - 2x$$ which is -1 --> so $$-1+A$$ must be positive hence A must be more than 1.

Now again: if A>1 then for any $$x$$ expression $$x^2 - 2x + A$$ is positive.

But if A=-15 (or any other number less than 1) we can find some $$x-es$$ for which expression $$x^2 - 2x + A$$ is not positive, so theese values of A (values of $$A\leq{1}$$) are not valid.

Hope it's clear.

I understood the approach but the fact which is baffling me is that say the equation after subsituting value of x=-3 i.e 15+ A > 0 now we can have a value of A=-3 or may be -4 etc and still have the value of the equation in statement 1 as +ve

Am I thinking too much or just lacking some thing basic concept.

I think you just don't understand one thing in statement (1): $$x^2-2x+A>0$$ FOR ALL $$x-es$$.

You say that if $$x=-3$$ then $$A$$ can be for example -10 (or any number more than -15) and $$x^2-2x+A$$ will be positive, $$but$$ if $$x=1$$ does $$A=-10$$ makes $$x^2-2x+A$$ positive? NO!

So you should find such value of $$A$$ (such range) for which $$x^2-2x+A$$ is positive no matter what value of $$x$$ you'll plug. And the way how to find this range is shown in my previous posts.
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16 Jul 2010, 09:23
Thanks Bunuel,

Now I understood,

I appreciate your patience in making me understand this one ..!

Cheers,
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27 Jul 2010, 14:47
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And all this has to come to me in less than 2 mins?
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10 Apr 2012, 18:17
Is A positive?

1) X^2-2X+A is positive for all X
2) AX^2 + 1 is positive for all X

given answer as A...but i thought it should be E..
source: hard problems from gmatclub tests number properties I
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10 Apr 2012, 20:03
1) X^2-2X+A is positive for all X
I think A could not be the answer, for example, if A = 0, and X = 4, then also the expression is positive, but A = 0 is neither positive nor negative
Again, if A = 1, and and X = 4, then also the expression is positive

2) AX^2 + 1 is positive for all X
Same logic as above, if A is 0, then the expression is positive, and the expression is also postive for any value of X where A > 0

In a nutshell, I too think the answer is E.

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11 Apr 2012, 09:42
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rongali wrote:
Is A positive?

1) X^2-2X+A is positive for all X
2) AX^2 + 1 is positive for all X

given answer as A...but i thought it should be E..
source: hard problems from gmatclub tests number properties I

1) X^2-2X+A is positive for all X

For all values of X,$$X^2-2X+A > 0$$
This means, for X = 0, $$X^2-2X+A > 0$$; for X = 1, $$X^2-2X+A > 0$$; for X = -2, $$X^2-2X+A > 0$$ etc etc etc

Let's put X = 0. $$0^2-2*0+A > 0$$ should hold. Therefore, A > 0 should hold.
Sufficient.

2) AX^2 + 1 is positive for all X

For all X, $$AX^2 + 1 > 0$$
Here, A could be positive or A could be 0 (since, when A = 0, we get 1 > 0 which holds no matter what the value of X.)
Since A can be 0, we cannot say whether A is positive. Not Sufficient.

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04 Jul 2012, 02:55
Bunuel wrote:
noboru wrote:
Is A positive?

x^2-2x+A is positive for all x
Ax^2+1 is positive for all x

OA is A

(2) $$Ax^2+1$$ is positive for all $$x$$:

$$Ax^2+1>0$$ --> when $$A\geq0$$ this expression is positive for all $$x$$. So $$A$$ can be zero too.

Not sufficient.

Why didn't you use the discriminant formula to assess statement 2?

I tried the discriminant rule and got a>0. I had 0-4a<0 which turns to a>0.

What am I missing here?

Thanks,
Diana
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04 Jul 2012, 02:59
dianamao wrote:
Bunuel wrote:
noboru wrote:
Is A positive?

x^2-2x+A is positive for all x
Ax^2+1 is positive for all x

OA is A

(2) $$Ax^2+1$$ is positive for all $$x$$:

$$Ax^2+1>0$$ --> when $$A\geq0$$ this expression is positive for all $$x$$. So $$A$$ can be zero too.

Not sufficient.

Why didn't you use the discriminant formula to assess statement 2?

I tried the discriminant rule and got a>0. I had 0-4a<0 which turns to a>0.

What am I missing here?

Thanks,
Diana

You are right: if we use the same approach for (2) then we'll get A>0 BUT if A=0 then Ax^2+1 won't be a quadratic function anymore. So this approach will work only if A doesn't equal to zero, but we can not eliminate this case and if A=0 then Ax^2+1=1 is also always positive. Hence Ax^2+1 is positive for A>0 (if we use quadratic function approach) as well as for A=0, so for $$A\geq0$$.

Hope it's clear.
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19 Aug 2012, 18:11
This is a question from gmat club tests.

Is A positive?
1. x^2 -2x +A is positive for all x.
2. A*x^2 +1 is positive for all x.

I got E and my way of solving is as below:
St1. x^2 - 2x +A > 0
Let x=0, so A>0. Let x=-1, so A>-3. In this case A can be negative or positive. Insufficient.

St2. A*x^2 +1 > 0
Let x=-1, so A>-1. Again A can be positive or negative. Insufficient.

St1+St2: Let x=-1, so A > -1. Again A can be positive or negative. Insufficient.

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20 Aug 2012, 00:36
shivamayam wrote:
This is a question from gmat club tests.

Is A positive?
1. x^2 -2x +A is positive for all x.
2. A*x^2 +1 is positive for all x.

I got E and my way of solving is as below:
St1. x^2 - 2x +A > 0
Let x=0, so A>0. Let x=-1, so A>-3. In this case A can be negative or positive. Insufficient.

St2. A*x^2 +1 > 0
Let x=-1, so A>-1. Again A can be positive or negative. Insufficient.

St1+St2: Let x=-1, so A > -1. Again A can be positive or negative. Insufficient.

Merging similar topics. Please refer to the solutions above.
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Re: Is A positive? x^2-2x+A is positive for all x Ax^2+1 is [#permalink]

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21 Aug 2012, 15:51
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best way to deal this problem is to bet on A more than X..
it wud b yes if A>0 Or No ,if A<0 ....
then first assume A>0 , then check whether statement 1 & 2 is true or not for all value of X....
then assume A<0 ,then check whether statement 1 & 2 is true or not for all value of X....
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21 Aug 2012, 18:57
dianamao wrote:
Bunuel wrote:
noboru wrote:
Is A positive?

x^2-2x+A is positive for all x
Ax^2+1 is positive for all x

OA is A

(2) $$Ax^2+1$$ is positive for all $$x$$:

$$Ax^2+1>0$$ --> when $$A\geq0$$ this expression is positive for all $$x$$. So $$A$$ can be zero too.

Not sufficient.

Why didn't you use the discriminant formula to assess statement 2?

I tried the discriminant rule and got a>0. I had 0-4a<0 which turns to a>0.

What am I missing here?

Thanks,
Diana

@Diana - Which discriminant rule did you use?
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Re: Is A positive?   [#permalink] 21 Aug 2012, 18:57

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