Possible Integer Values of k for Quadratic Equations

CAT 2022 Slot 3 · QA · Medium · Algebra

Suppose kk is any integer such that the equation 2x2+kx+5=02x^2 + kx + 5 = 0 has no real roots and the equation x2+(k5)x+1=0x^2 + (k - 5)x + 1 = 0 has two distinct real roots for xx. Then, the number of possible values of kk is

  1. A.

    7

  2. B.

    8

  3. C.

    9

  4. D.

    13

Answer

C

Explanation

For 2x2+kx+5=02x^2 + kx + 5 = 0 to have no real roots, its discriminant must be negative: D1=k24(2)(5)<0    k2<40    40<k<40D_1 = k^2 - 4(2)(5) < 0 \implies k^2 < 40 \implies -\sqrt{40} < k < \sqrt{40} Since kk is an integer, k{6,5,,5,6}k \in \{-6, -5, \dots, 5, 6\}.

For x2+(k5)x+1=0x^2 + (k - 5)x + 1 = 0 to have two distinct real roots, its discriminant must be positive: D2=(k5)24(1)(1)>0    (k5)2>4D_2 = (k - 5)^2 - 4(1)(1) > 0 \implies (k - 5)^2 > 4     k5>2 or k5<2    k>7 or k<3\implies k - 5 > 2 \text{ or } k - 5 < -2 \implies k > 7 \text{ or } k < 3

Combining both conditions for integer kk: k{6,5,4,3,2,1,0,1,2}k \in \{-6, -5, -4, -3, -2, -1, 0, 1, 2\} There are 9 such integer values for kk.

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