Minimum Value of Quotient Function

XAT 2021 · Quantitative Ability & Data Interpretation · Hard · Algebra

Let f(x)=x2+1x2−1f(x) = \frac{x^2+1}{x^2-1} if x≠1,−1x \neq 1, -1, and 11 if x=1,−1x = 1, -1. Let g(x)=x+1x−1g(x) = \frac{x+1}{x-1} if x≠1x \neq 1, and 33 if x=1x = 1. What is the minimum possible values of f(x)g(x)\frac{f(x)}{g(x)}?

  1. A.

    12\frac{1}{2}

  2. B.

    -1

  3. C.

    14\frac{1}{4}

  4. D.

    13\frac{1}{3}

  5. E.

    1

Answer

D

Explanation

For x≠1,−1x \neq 1, -1: f(x)g(x)=(x2+1)/(x2−1)(x+1)/(x−1)=x2+1(x+1)2\frac{f(x)}{g(x)} = \frac{(x^2+1)/(x^2-1)}{(x+1)/(x-1)} = \frac{x^2+1}{(x+1)^2}. Let y=x2+1(x+1)2  ⟹  y(x2+2x+1)=x2+1  ⟹  x2(y−1)+2yx+(y−1)=0y = \frac{x^2+1}{(x+1)^2} \implies y(x^2+2x+1) = x^2+1 \implies x^2(y-1) + 2yx + (y-1) = 0. Discriminant D≥0  ⟹  4y2−4(y−1)2≥0  ⟹  4y2−4(y2−2y+1)≥0  ⟹  8y−4≥0  ⟹  y≥12D \ge 0 \implies 4y^2 - 4(y-1)^2 \ge 0 \implies 4y^2 - 4(y^2 - 2y + 1) \ge 0 \implies 8y - 4 \ge 0 \implies y \ge \frac{1}{2}. Check special points: For x=1x = 1: f(1)=1f(1) = 1, g(1)=3  ⟹  f(1)g(1)=13g(1) = 3 \implies \frac{f(1)}{g(1)} = \frac{1}{3}. For x=−1x = -1: f(−1)=1f(-1) = 1, g(−1)=0g(-1) = 0, undefined. Since 13<12\frac{1}{3} < \frac{1}{2}, the minimum possible value is 13\frac{1}{3}.

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