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I solved this (slightly) more algebraically in the sense that: -9 < x < 9 and -5<y<5 so multiplying the inequality of y by -2 we get

10 > -2y > -10, flipping the inequality for x around the same way we get 9 > x > -9 now adding them we obtain

19 > x - 2y > -19, and we see that the biggest prime number in this range is 17
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imperfectdark
I solved this (slightly) more algebraically in the sense that: -9 < x < 9 and -5<y<5 so multiplying the inequality of y by -2 we get

10 > -2y > -10, flipping the inequality for x around the same way we get 9 > x > -9 now adding them we obtain

19 > x - 2y > -19, and we see that the biggest prime number in this range is 17

why cant we solve as below (i know its wrong coz of the -2 multiplication)
-9<x<9
-10<2y<10

subtract

1<x-2y<-1

is it becoz the -2 in x-2y creates the problem while subtraction or we cannot subtract inequalities with same sign?? pls help
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imperfectdark
I solved this (slightly) more algebraically in the sense that: -9 < x < 9 and -5<y<5 so multiplying the inequality of y by -2 we get

10 > -2y > -10, flipping the inequality for x around the same way we get 9 > x > -9 now adding them we obtain

19 > x - 2y > -19, and we see that the biggest prime number in this range is 17

why cant we solve as below (i know its wrong coz of the -2 multiplication)
-9<x<9
-10<2y<10

subtract

1<x-2y<-1

is it becoz the -2 in x-2y creates the problem while subtraction or we cannot subtract inequalities with same sign?? pls help


Hey,

If inequalities have same sign we can perform only addition operation.
Subtraction is allowed only when they have different sign and the result will have the sign of the inequality from which we are subtracting the other one.

for example, in this case:

-9<x<9---- 1
-10<2y<10---2

From 2==> 10>2y>-10

Subtracting 2 from 1 and the sign of the resulting inequality will be that of Ineq1:

Hence, -19<x-2y<19
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Bunuel
If \(x^2 \lt 81\) and \(y^2 \lt 25\), what is the largest prime number that can be equal to \(x-2y\)?

A. 3
B. 7
C. 11
D. 13
E. 17

\(x^2 \lt 81\) and \(y^2 \lt 25\) should help you understand that x is less than 9 and y is less than 5. But also note that x can be between -9 and 9 but not inclusive. Same is the case with y. -5<y<5.

\(x-2y\)= Prime Number

One thing is obvious we have to choose a negative value of y.

13 is easy to achieve. 7-(2*-3)=13.

But why we should lower x even though we can treat x as 8. Can we achieve 17? Remember that neither is supposed to be an integer.

8-(2*-4.5)
8+9
17

We can achieve 17. So answer is 17.

Note: Even if you are unable to figure out the value you should understand that it is achievable and should go ahead with option E.
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Hello, TheNightKing. I agree with your logic, all except for a statement that was probably typed too fast:

TheNightKing
One thing is obvious we have to choose a negative value of x.
I think you meant a negative value of y. (I told you I would read your posts.)

Cheers,
Andrew
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MentorTutoring
Hello, TheNightKing. I agree with your logic, all except for a statement that was probably typed too fast:

TheNightKing
One thing is obvious we have to choose a negative value of x.
I think you meant a negative value of y.
MentorTutoring
Hey Andrew, Thanks for pointing out. I updated the post. :)

Quote:

(I told you I would read your posts.)

Cheers,
Andrew

That's one of the reasons I post every day since I know there is at least one reader out there who will appreciate my work. :please
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I think this is a high-quality question and I agree with explanation.
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I think this is a high-quality question and I agree with explanation. Solved this for the second time and still got it wrong. Goes to show how important it is to read the question and understand whether x and y need to be integers or not.
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Hi
I didn't understand how did we determine the value of y can someone please explain how can we choose the value of y
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If \(x^2 \lt 81\) and \(y^2 \lt 25\), what is the largest prime number that can be equal to \(x-2y\)?

A. 3
B. 7
C. 11
D. 13
E. 17

Hi
I didn't understand how did we determine the value of y can someone please explain how can we choose the value of y

\(x^2 \lt 81\) means that \(-9 \lt x \lt 9\);

\(y^2 \lt 25\) means that \(-5 \lt y \lt 5\). Multiply by 2: \(-10 \lt 2y \lt 10\).

To find the largest value of \(x-2y\), maximize x and minimize 2y:

(almost 9) - (almost -10) = (almost 9) + (almost 10) = (almost 19).

The actual value is less than 19, which means that the largest prime that can be equal to \(x-2y\) is 17. For example: \(x=8\) and \(y=-4.5\).

Hope it's clear.
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I have edited the question and the solution by adding more details to enhance its clarity. I hope it is now easier to understand.
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I think this is a high-quality question and I agree with explanation.
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Bunuel
If \(x^2 < 81\) and \(y^2 < 25\), what is the largest prime number that can be equal to \(x - 2y\)?

A. 3
B. 7
C. 11
D. 13
E. 17

Bunuel
chetan2u
I am facing difficulty in grasping one concept in similar question type. In GMAT \(\sqrt{x^2} \)= |x|. For example \(\sqrt{25}\)=5. not -5 or 5. So applying the same concept in this question x should be less than 9 and not -9<x<9.

I am missing something, i would really appreciate we you could help me find me my mistake. Thank you
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Bunuel
If \(x^2 < 81\) and \(y^2 < 25\), what is the largest prime number that can be equal to \(x - 2y\)?

A. 3
B. 7
C. 11
D. 13
E. 17

Bunuel
chetan2u
I am facing difficulty in grasping one concept in similar question type. In GMAT \(\sqrt{x^2} \)= |x|. For example \(\sqrt{25}\)=5. not -5 or 5. So applying the same concept in this question x should be less than 9 and not -9<x<9.

I am missing something, i would really appreciate we you could help me find me my mistake. Thank you

Hi
On GMAT, square root is always positive, so \(\sqrt{25}=5\).
But when we write \(x^2=25\), x could be 5 or -5.
The difference is we are taking square root in one and in other we are talking of square.
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Bunuel
If \(x^2 < 81\) and \(y^2 < 25\), what is the largest prime number that can be equal to \(x - 2y\)?

A. 3
B. 7
C. 11
D. 13
E. 17

\(x^2 < 81\)
\(x^2 - 9^2 < 0\)
(x+9)(x-9) < 0
-9 < x < 9

Similarly,
\(y^2 < 25\)
-5 < y < 5


Since x is less than 9 and 2y is greater than -10, (x - 2y) is less than 19 for sure. The greatest prime number less than 19 is 17. Is 17 a possible value of x - 2y? Yes

x can be 8 (largest positive integer possible) and y can be -4.5 so that 2y = -9 which is an integer.

\(x - 2y = 8 - (-9) = 17\) (a prime)

This must be the answer.

Answer (E)

Note that x - 2y can take a value of 18 (when x = 8.8 and y = - 4.6) too but that is not prime.
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I like the solution - it’s helpful.
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I like the solution - it’s helpful.
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