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inequality

by nafiul9090 » Wed Apr 18, 2012 6:50 am
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

please help
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by aneesh.kg » Wed Apr 18, 2012 7:39 am
The question does not say that X and Y has to be positive, or integers.
Therefore,
-9 < X < 9, -5 < Y < 5

The options are arranged in an ascending order of prime numbers, so let's try them in the reverse order to find the largest prime number.

Is 17 possible?
17 is possible when X = 8.5 , Y = -4.25 (and many such values) because (8.5) - 2(-4.25).
So, (E) is the answer.
Last edited by aneesh.kg on Wed Apr 18, 2012 7:50 am, edited 6 times in total.
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by bryan88 » Wed Apr 18, 2012 7:39 am
-9<X<9
-5<Y<5

Check if 17 is possible
If
X=8
Y=-4.5

Hence E
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by nafiul9090 » Wed Apr 18, 2012 8:43 am
yes the answer is E
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by Birottam Dutta » Wed Apr 18, 2012 8:47 am
You have to understand that the trick here is to notice that x and y need not be positive or integers and then proceed. The answer comes easily then!
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by sam2304 » Wed Apr 18, 2012 9:41 am
Good one, I missed the catch :( Its so simple and doesn't take much time solving the problem once you understand the catch.
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by nafiul9090 » Wed Apr 18, 2012 4:07 pm
Birottam Dutta wrote:You have to understand that the trick here is to notice that x and y need not be positive or integers and then proceed. The answer comes easily then!
i missed that part while answering the question...:-(
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