Continued Fraction Evaluation

CAT 2004 Slot 1 · QA · Medium · Algebra

Let y=12+13+12+13+y = \frac{1}{2 + \frac{1}{3 + \frac{1}{2 + \frac{1}{3 + \dots}}}} What is the value of yy?

  1. A.

    11+32\frac{\sqrt{11} + 3}{2}

  2. B.

    1132\frac{\sqrt{11} - 3}{2}

  3. C.

    15+32\frac{\sqrt{15} + 3}{2}

  4. D.

    1532\frac{\sqrt{15} - 3}{2}

Answer

D

Explanation

We can express yy recursively as: y=12+13+y=3+y2(3+y)+1=3+y7+2yy = \frac{1}{2 + \frac{1}{3 + y}} = \frac{3 + y}{2(3 + y) + 1} = \frac{3 + y}{7 + 2y} y(7+2y)=3+y    2y2+6y3=0y(7 + 2y) = 3 + y \implies 2y^2 + 6y - 3 = 0 Using the quadratic formula: y=6±364(2)(3)4=6±604=3±152y = \frac{-6 \pm \sqrt{36 - 4(2)(-3)}}{4} = \frac{-6 \pm \sqrt{60}}{4} = \frac{-3 \pm \sqrt{15}}{2} Since y>0y > 0, y=1532y = \frac{\sqrt{15} - 3}{2}.

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