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Quadratic Equation and Inequalities question

2021 · 25 Feb · Shift 1 · Q27
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  5. /2021 · 25 Feb · Shift 1 · Q27

Quadratic Equation and Inequalities question

2021 · 25 Feb · Shift 1 · Q27

JEE MainMathematicsQuadratic Equation and InequalitiesMCQ+4 / −1
The integer 'k', for which the inequality x2 −-− 2(3k −-− 1)x + 8k2 −-− 7 > 0 is valid for every x in R, is :
  1. A
    4
  2. B
    2
  3. C
    3
  4. D
    0
View written solutionFree

Correct answer: C

  1. We are given the quadratic inequality

x2−2(3k−1)x+8k2−7>0x^2 - 2(3k-1)x + 8k^2 - 7 > 0x2−2(3k−1)x+8k2−7>0

which must be valid for every x∈Rx \in \mathbb{R}x∈R.

  1. For a quadratic

ax2+bx+c>0for all x∈R,ax^2+bx+c>0 \quad \text{for all } x\in\mathbb{R},ax2+bx+c>0for all x∈R,

the necessary and sufficient conditions are:

  • a>0a>0a>0
  • discriminant Δ<0\Delta<0Δ<0

Here,

a=1>0a=1>0a=1>0

so we only need:

Δ<0.\Delta<0.Δ<0.

  1. Identify coefficients:

a=1,b=−2(3k−1),c=8k2−7.a=1,\quad b=-2(3k-1),\quad c=8k^2-7.a=1,b=−2(3k−1),c=8k2−7.

Thus,

Δ=b2−4ac=[−2(3k−1)]2−4(1)(8k2−7).\Delta=b^2-4ac=[-2(3k-1)]^2-4(1)(8k^2-7).Δ=b2−4ac=[−2(3k−1)]2−4(1)(8k2−7).

  1. Simplify:

[−2(3k−1)]2=4(3k−1)2[-2(3k-1)]^2=4(3k-1)^2[−2(3k−1)]2=4(3k−1)2

So,

Δ=4(3k−1)2−4(8k2−7)\Delta=4(3k-1)^2-4(8k^2-7)Δ=4(3k−1)2−4(8k2−7)

=4[(3k−1)2−(8k2−7)].=4\big[(3k-1)^2-(8k^2-7)\big].=4[(3k−1)2−(8k2−7)].

Now,

(3k−1)2=9k2−6k+1(3k-1)^2=9k^2-6k+1(3k−1)2=9k2−6k+1

Hence,

Δ=4[(9k2−6k+1)−8k2+7]\Delta=4\big[(9k^2-6k+1)-8k^2+7\big]Δ=4[(9k2−6k+1)−8k2+7]

=4(k2−6k+8)=4(k^2-6k+8)=4(k2−6k+8)

=4(k−2)(k−4).=4(k-2)(k-4).=4(k−2)(k−4).

  1. For positivity for all real xxx:

Δ<0⇒4(k−2)(k−4)<0\Delta<0 \Rightarrow 4(k-2)(k-4)<0Δ<0⇒4(k−2)(k−4)<0

⇒(k−2)(k−4)<0.\Rightarrow (k-2)(k-4)<0.⇒(k−2)(k−4)<0.

This holds when

2<k<4.2<k<4.2<k<4.

  1. Since kkk is an integer, the only possible value is

k=3.k=3.k=3.

  1. Check options:
  • A: 444 → gives Δ=0\Delta=0Δ=0, not strictly positive for all xxx
  • B: 222 → gives Δ=0\Delta=0Δ=0, not strictly positive for all xxx
  • C: 333 → valid
  • D: 000 → gives Δ>0\Delta>0Δ>0, not valid

Therefore, the correct option is:

3\boxed{3}3​

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