Sum of Real Roots of Polynomial Equation

CAT 2025 Slot 3 · Quantitative Ability · Hard · Algebra

This is a hard Quantitative Ability question from the CAT 2025 Slot 3 paper. It tests Algebra. The full answer key and a step-by-step explanation are below — try it yourself first, then reveal the solution.

If f(x)=(x2+3x)(x2+3x+2)f(x) = (x^2 + 3x)(x^2 + 3x + 2) then the sum of all real roots of the equation f(x)+1=9701\sqrt{f(x) + 1} = 9701, is

  1. A.

    -6

  2. B.

    6

  3. C.

    3

  4. D.

    -3

Answer

D

Explanation

Let k=x2+3xk = x^2 + 3x. Then f(x)=k(k+2)=k2+2kf(x) = k(k+2) = k^2 + 2k. f(x)+1=k2+2k+1=(k+1)2f(x) + 1 = k^2 + 2k + 1 = (k+1)^2. f(x)+1=k+1=9701\sqrt{f(x) + 1} = |k+1| = 9701. Since k+1=x2+3x+1k+1 = x^2 + 3x + 1, we have x2+3x+1=9701x^2 + 3x + 1 = 9701 or x2+3x+1=9701x^2 + 3x + 1 = -9701.

  1. x2+3x9700=0x^2 + 3x - 9700 = 0: Discriminant D=9+4(9700)>0D = 9 + 4(9700) > 0, real roots exist. Sum of roots = 3/1=3-3/1 = -3.
  2. x2+3x+9702=0x^2 + 3x + 9702 = 0: Discriminant D=94(9702)<0D = 9 - 4(9702) < 0, no real roots. Thus, the sum of all real roots is 3-3.

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