Polynomial Roots and Concentric Circles Area Ratio

XAT 2024 · Quantitative Ability & Data Interpretation · Medium · Algebra

The roots of the polynomial P(x)=2x3−11x2+17x−6P(x) = 2x^3 - 11x^2 + 17x - 6 are the radii of three concentric circles.

The ratio of their area, when arranged from the largest to the smallest, is:

  1. A.

    6:2:1

  2. B.

    9:4:1

  3. C.

    16:6:3

  4. D.

    36:16:1

  5. E.

    None of the remaining options is correct.

Answer

D

Explanation

Let's find the roots of P(x)=2x3−11x2+17x−6=0P(x) = 2x^3 - 11x^2 + 17x - 6 = 0. By inspection, P(2)=2(8)−11(4)+17(2)−6=16−44+34−6=0P(2) = 2(8) - 11(4) + 17(2) - 6 = 16 - 44 + 34 - 6 = 0. Dividing P(x)P(x) by (x−2)(x-2) gives 2x2−7x+3=02x^2 - 7x + 3 = 0. (2x−1)(x−3)=0  ⟹  x=1/2,3(2x - 1)(x - 3) = 0 \implies x = 1/2, 3. So the three roots (radii) are r1=3,r2=2,r3=1/2r_1 = 3, r_2 = 2, r_3 = 1/2. The areas of the circles are proportional to r2r^2. Ratio of areas =32:22:(1/2)2=9:4:1/4=36:16:1= 3^2 : 2^2 : (1/2)^2 = 9 : 4 : 1/4 = 36 : 16 : 1.

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