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SaquibHGMATWhiz
twobagels
Both roots of the quadratic equation \(x^2 - 63*x + k = 0\) are prime numbers. The number of possible values of k are?
A. 0
B. 1
C. 2
D. 3
E. 4 or more
Solution:

  • Let the roots of \(x^2 - 63x + k = 0\) be \(P_1\) and \(P_2\)
  • So, we can infer \(P_1+P_2=63\)

  • Sum of two primes will be 63 (odd) iff one of them is 2 (even)

  • So, \(P_1=2\) and \(P_2=61\) are the only roots possible
  • For one pair of x, we will have only one value of k


Hence the right answer is Option B

What does it mean "for one pair of x" ?
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Given: Both roots of the quadratic equation \(x^2 - 63*x + k = 0\) are prime numbers.
Asked: The number of possible values of k are?

Let a & b be the roots of the equation \(x^2 - 63*x + k = 0\)
a + b = 63
ab=k

Possible values of (a, b) = (2,61)
Since sum a+b is odd, one of the numbers must be even and another odd and since both are prime numbers
Only even 2 is a prime number.

IMO B
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