Integer Solutions to Prime Equation

XAT 2023 · Quantitative Ability & Data Interpretation · Hard · Number Theory

Let xx and yy be two positive integers and pp be a prime number. If x(x−p)−y(y+p)=7px(x - p) - y(y + p) = 7p, what will be the minimum value of x−yx - y?

  1. A.

    1

  2. B.

    3

  3. C.

    5

  4. D.

    7

  5. E.

    None of the above

Answer

E

Explanation

Expand equation: x2−px−y2−yp=7p  ⟹  (x2−y2)−p(x+y)=7p  ⟹  (x−y−p)(x+y)=7px^2 - px - y^2 - yp = 7p \implies (x^2 - y^2) - p(x + y) = 7p \implies (x - y - p)(x + y) = 7p. Since x,yx, y are positive integers, x+y>x−y−px+y > x-y-p. Case 1: x−y−p=1,x+y=7px - y - p = 1, x + y = 7p. Case 2: x−y−p=p,x+y=7x - y - p = p, x + y = 7. Case 3: x−y−p=7,x+y=px - y - p = 7, x + y = p. Checking integer conditions yields no consistent positive integer solution for x,yx, y with pp prime.

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