Cubic Polynomial Real Roots Condition

CAT 2023 Slot 1 · QA · Hard · Algebra

The equation x3+(2r+1)x2+(4r1)x+2=0x^3 + (2r+1)x^2 + (4r-1)x + 2 = 0 has 2-2 as one of the roots. If the other two roots are real, then the minimum possible non-negative integer value of rr is

Answer

2

Explanation

Since x=2x = -2 is a root, factor out (x+2)(x + 2): x3+(2r+1)x2+(4r1)x+2=(x+2)(x2+(2r1)x+1)=0x^3 + (2r+1)x^2 + (4r-1)x + 2 = (x + 2)(x^2 + (2r - 1)x + 1) = 0

For the remaining quadratic equation x2+(2r1)x+1=0x^2 + (2r - 1)x + 1 = 0 to have real roots, its discriminant must be non-negative: D=(2r1)24(1)(1)0D = (2r - 1)^2 - 4(1)(1) \ge 0 (2r1)24    2r12(2r - 1)^2 \ge 4 \implies |2r - 1| \ge 2 This gives two cases:

  1. 2r12    2r3    r1.52r - 1 \ge 2 \implies 2r \ge 3 \implies r \ge 1.5
  2. 2r12    2r1    r0.52r - 1 \le -2 \implies 2r \le -1 \implies r \le -0.5

We need the minimum possible non-negative integer value of rr. For non-negative integers (r0r \ge 0), r1.5r \ge 1.5 gives the smallest integer r=2r = 2.

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