Pair of non-linear equations

CAT 2021 Slot 2 · QA · Medium · Quadratic Equations

Consider the pair of equations: x2xyx=22x^2 - xy - x = 22 and y2xy+y=34y^2 - xy + y = 34. If x>yx > y, then xyx - y equals

  1. A.

    8

  2. B.

    6

  3. C.

    7

  4. D.

    4

Answer

A

Explanation

Equation 1: x(xy1)=22x(x - y - 1) = 22 Equation 2: y(yx+1)=34    y(xy1)=34y(y - x + 1) = 34 \implies -y(x - y - 1) = 34

Subtracting Equation 2 from Equation 1: x2xyx(y2xy+y)=2234x^2 - xy - x - (y^2 - xy + y) = 22 - 34 x2y2(x+y)=12x^2 - y^2 - (x + y) = -12 (x+y)(xy)(x+y)=12(x + y)(x - y) - (x + y) = -12 (x+y)(xy1)=12(x + y)(x - y - 1) = -12

Also, adding Equation 1 and Equation 2: x(xy1)y(xy1)=22+34=56x(x - y - 1) - y(x - y - 1) = 22 + 34 = 56 (xy)(xy1)=56(x - y)(x - y - 1) = 56

Let k=xyk = x - y. Then: k(k1)=56k(k - 1) = 56 k2k56=0k^2 - k - 56 = 0 (k8)(k+7)=0(k - 8)(k + 7) = 0

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

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