Written on a blackboard is the polynomial . Calvin and Peter take turns alternatively (starting with Calvin) in the following game. During his turn, Calvin should either increase or decrease the coefficient of x by 1 , and during his turn, Peter should either increase or decrease the constant coefficient by 1 . Calvin wins if at any point of time the polynomial on the blackboard at that instant has integer roots. Which of the following is true.
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We can modify the equation x 2 + x + 2 0 1 4 with this: x 2 + C x + 2 0 1 4 + P , where C is the total sum of Calvin and P the sum of Peter. According to the equation, the roots are:
x = 2 − C ± C 2 − 8 0 5 6 − 4 P = − 2 C ± 2 C 2 − 8 0 5 6 − 4 P
Note that C must be a multiple of 2 ( 2 n ) and C 2 − 8 0 5 6 − 4 P must be a perfect square ( ( a n ) 2 ), where a is a parameter (both a and n are integers too); with this, the roots are modified into this
x = − 2 2 n ± 2 ( 2 n ) 2 − 8 0 5 6 − 4 P = − n ± n 2 − 2 0 1 4 − P
Given that C 2 − 8 0 5 6 − 4 P or n 2 − 2 0 1 4 − P must be a perfect square ( ( a n ) 2 ), then
n 2 − 2 0 1 4 − P = ( a n ) 2 ⟶ P = ( 1 − a 2 ) n 2 − 2 0 1 4 .
If we substitute this in the roots
− n ± a n ⟶ − n ( 1 + a ) o r n ( a − 1 ) , given that C = 2 n , then the roots are − 2 C ( 1 + a ) or 2 C ( a − 1 )
With this (and the note below), Calvin could win in a finite number of steps.
Note: Look that P + 2 0 1 4 = ( 1 − a 2 ) n 2 , you can substitute this in the modified quadratic equation and you´ll get this
x 2 + C x + 2 0 1 4 + P = x 2 + 2 n x + ( 1 − a 2 ) n 2
Solving the quadratic formula you can get the roots written above. You´ll get two integer solutions if a = 0 .