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another gud PS prob --- value of XY

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by Ian Stewart » Thu Oct 09, 2008 9:59 am
-4 < x < 7
-6 < y < 3

Just work with the extreme values- there are four products xy to consider:

lowest*lowest = (-4)*(-6) = 24
highest*highest = 7*3 = 21
lowest*highest = (-4)*3 = -12
highest*lowest = 7*(-6) = -42

So xy will lie between -42 and 24.
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by vivek.kapoor83 » Thu Oct 09, 2008 10:02 am
Thanks


How u arrived on this...can u pls tell me with a proper solution because.. I thot this way bt..i dont know how to prove it...Can u pls elaborate.
this sol. doesnt satisfy me although OA is ok

Coz i was thinking on he lines lke. multiply 2 values for which xy are less (-4,-6)and 2 values for which xy are greater.(7,3)
So. 24<xy<21..
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by Gmatss » Thu Oct 09, 2008 10:52 am
all you have to do for this problem is find the maximum possible value of xy and the minimum value of xy
if you line them up togather

-4 < x < 7
-6 < y < 3

The maximum possible value is -4 * -6 which is 24.
The least possible value is the greatest value from either x or y and the least value for x or y. In this case 7*-6 = -42 or 3 * -4 which is -12
hence -42 to 24.

I think what you did is just multiply the two least and two greatest but you hvae to consider (Greast * Least) (least * Greatest)
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