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Official Quant Thread for CAT 2012 Quantitative

Please continue here for all the Quant queries and discussions.
The links to the previous threads are:
Part 1
http://www.pagalguy.com/forum/quantitative-questions-and-answers/74662-official-quant-thread-cat-2012-a.html
Part 2
http://www.pagalguy.com/forum/quantitative-questions-and...
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If F(x) = x^4 -360x^2 +400, then for x E Integer, if F(x) is a prime number, then what is the sum of all possible F(x)??



f(x) = x^4 - 360x^2 + 400 = (x^2 + 20)^2 - 400x^2

=> f(x) = (x^2 + 20 - 20x)*(x^2 + 20 + 20x)

For f(x) to be prime => One of x^2 - 20x + 20 and x^2 + 20x + 20 should be equal to one and the other must be a prime number


Case 1:
Let x^2 - 20x + 20 = 1
=> x^2 - 20x + 19 = 0

=> x = 1, 19

=> for x = 1 => x^2 + 20x + 20 = 41 a prime number
=> for x = 19 => 761 a prime number


Case 2:
Let x^2 + 20x + 20 = 1
=> x^2 + 20x + 19 = 0

=> x = -1, -19

=> for x = -1 => x^2 - 20x + 20 = 41 a prime number
=> for x = -19 => 761 a prime number

So 41 and 761 are possible

=> Sum = 802
Smoothest seas do not make tough sailors
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