Let x , y , z be three non-negative integers such that x + y + z = 1 0 . The maximum possible value of x y z + x y + y z + z x is:
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Cool! Its a KVPY 2013 problem! And I did it the same way there!
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I tried it first time pranal here only . But going to solve KVPY paper 2013 tomorrow as KVPY is on 2 November.
Note that the first condition is equivalent to ( x + 1 ) + ( y + 1 ) + ( z + 1 ) = 1 3
To answer the question would be the same as finding the maximum possible value of
x y z + x y + y z + z x + x + y + z + 1
= x y z + x y + y x + z x + 1 1
= ( x + 1 ) ( y + 1 ) ( z + 1 ) , with ( x + 1 ) , ( y + 1 ) , ( z + 1 ) all being positive integers.
Now assume w.l.o.g. that x ≤ y ≤ z or equivalently x + 1 ≤ y + 1 ≤ z + 1 .
Checking for the cases x + 1 = 1 , 2 , … , 4 (since if x ≥ 4 , we have x + y + z ≥ 1 2 , a contradiction), we have the case x = 3 , y = 3 , z = 4 giving a maximum possible value for x y z + x y + y z + z x as ( 3 + 1 ) ( 3 + 1 ) ( 4 + 1 ) − 1 1 = 6 9
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For max. Value all must be around same values ( what I apply to All problems) thus I tried 3,3,4 and there is was. :)