Pair of non-linear equations

CAT 2021 Slot 2 · Quantitative Ability · Medium · Quadratic Equations

Consider the pair of equations: x2−xy−x=22x^2 - xy - x = 22 and y2−xy+y=34y^2 - xy + y = 34. If x>yx > y, then x−yx - y equals

  1. A.

    8

  2. B.

    6

  3. C.

    7

  4. D.

    4

Answer

A

Explanation

Equation 1: x(x−y−1)=22x(x - y - 1) = 22 Equation 2: y(y−x+1)=34  ⟹  −y(x−y−1)=34y(y - x + 1) = 34 \implies -y(x - y - 1) = 34

Subtracting Equation 2 from Equation 1: x2−xy−x−(y2−xy+y)=22−34x^2 - xy - x - (y^2 - xy + y) = 22 - 34 x2−y2−(x+y)=−12x^2 - y^2 - (x + y) = -12 (x+y)(x−y)−(x+y)=−12(x + y)(x - y) - (x + y) = -12 (x+y)(x−y−1)=−12(x + y)(x - y - 1) = -12

Also, adding Equation 1 and Equation 2: x(x−y−1)−y(x−y−1)=22+34=56x(x - y - 1) - y(x - y - 1) = 22 + 34 = 56 (x−y)(x−y−1)=56(x - y)(x - y - 1) = 56

Let k=x−yk = x - y. Then: k(k−1)=56k(k - 1) = 56 k2−k−56=0k^2 - k - 56 = 0 (k−8)(k+7)=0(k - 8)(k + 7) = 0

Since x>yx > y, k=x−y>0k = x - y > 0, so x−y=8x - y = 8.

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