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Probability question

2022 · 25 Jul · Shift 1 · Q35
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  5. /2022 · 25 Jul · Shift 1 · Q35

Probability question

2022 · 25 Jul · Shift 1 · Q35

JEE MainMathematicsProbabilityMCQ+4 / −1
If the numbers appeared on the two throws of a fair six faced die are α\alphaα and β\betaβ, then the probability that x2+αx+β>0x^{2}+\alpha x+\beta\gt 0x2+αx+β>0, for all x∈Rx \in \mathbf{R}x∈R, is :
  1. A
    1736\frac{17}{36}3617​
  2. B
    49\frac{4}{9}94​
  3. C
    12\frac{1}{2}21​
  4. D
    1936\frac{19}{36}3619​
View written solutionFree

Correct answer: A

  1. We need the quadratic f(x)=x2+αx+βf(x)=x^2+\alpha x+\betaf(x)=x2+αx+β to satisfy f(x)>0for all x∈R.f(x)>0 \quad \text{for all } x\in \mathbb{R}.f(x)>0for all x∈R.

  2. For a quadratic ax2+bx+cax^2+bx+cax2+bx+c to be positive for all real xxx, we need:

  • a>0a>0a>0, and
  • discriminant <0<0<0.

Here, a=1>0a=1>0a=1>0, so we only need α2−4β<0.\alpha^2-4\beta<0.α2−4β<0. That is, α2<4β.\alpha^2<4\beta.α2<4β.

  1. Since α\alphaα and β\betaβ are outcomes of two throws of a fair die, α,β∈{1,2,3,4,5,6}.\alpha,\beta\in\{1,2,3,4,5,6\}.α,β∈{1,2,3,4,5,6}. Total number of ordered pairs is 6×6=36.6\times 6=36.6×6=36.

  2. Count pairs (α,β)(\alpha,\beta)(α,β) satisfying α2<4β.\alpha^2<4\beta.α2<4β. We check each possible value of α\alphaα:

  • If α=1\alpha=1α=1: 1<4β1<4\beta1<4β true for all β=1,2,3,4,5,6\beta=1,2,3,4,5,6β=1,2,3,4,5,6. Count =6=6=6.

  • If α=2\alpha=2α=2: 4<4β  ⟺  β>14<4\beta \iff \beta>14<4β⟺β>1 so β=2,3,4,5,6\beta=2,3,4,5,6β=2,3,4,5,6. Count =5=5=5.

  • If α=3\alpha=3α=3: 9<4β  ⟺  β>949<4\beta \iff \beta>\frac{9}{4}9<4β⟺β>49​ so β=3,4,5,6\beta=3,4,5,6β=3,4,5,6. Count =4=4=4.

  • If α=4\alpha=4α=4: 16<4β  ⟺  β>416<4\beta \iff \beta>416<4β⟺β>4 so β=5,6\beta=5,6β=5,6. Count =2=2=2.

  • If α=5\alpha=5α=5: 25<4β  ⟺  β>254=6.2525<4\beta \iff \beta>\frac{25}{4}=6.2525<4β⟺β>425​=6.25 impossible. Count =0=0=0.

  • If α=6\alpha=6α=6: 36<4β  ⟺  β>936<4\beta \iff \beta>936<4β⟺β>9 impossible. Count =0=0=0.

  1. Total favorable cases: 6+5+4+2+0+0=17.6+5+4+2+0+0=17.6+5+4+2+0+0=17.

  2. Therefore, the required probability is 1736.\frac{17}{36}.3617​.

  3. Comparing with the stored correct answer: stored answer is A, i.e. 1736\frac{17}{36}3617​, which matches.

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