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Quadratic Equation and Inequalities question

2024 · 9 Apr · Shift 1 · Q42
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  5. /2024 · 9 Apr · Shift 1 · Q42

Quadratic Equation and Inequalities question

2024 · 9 Apr · Shift 1 · Q42

JEE MainMathematicsQuadratic Equation and InequalitiesMCQ+4 / −1
Let α,β\alpha, \betaα,β be the roots of the equation x2+22x−1=0x^2+2 \sqrt{2} x-1=0x2+22​x−1=0. The quadratic equation, whose roots are α4+β4\alpha^4+\beta^4α4+β4 and 110(α6+β6)\frac{1}{10}(\alpha^6+\beta^6)101​(α6+β6), is:
  1. A
    x2−180x+9506=0x^2-180 x+9506=0x2−180x+9506=0
  2. B
    x2−195x+9506=0x^2-195 x+9506=0x2−195x+9506=0
  3. C
    x2−190x+9466=0x^2-190 x+9466=0x2−190x+9466=0
  4. D
    x2−195x+9466=0x^2-195 x+9466=0x2−195x+9466=0
View written solutionFree

Correct answer: B

  1. Given quadratic and basic symmetric sums

The roots α,β\alpha,\betaα,β satisfy x2+22x−1=0.x^2+2\sqrt{2}x-1=0.x2+22​x−1=0.

So, α+β=−22,αβ=−1.\alpha+\beta=-2\sqrt{2}, \qquad \alpha\beta=-1.α+β=−22​,αβ=−1.

We need the quadratic whose roots are r1=α4+β4,r2=110(α6+β6).r_1=\alpha^4+\beta^4, \qquad r_2=\frac{1}{10}(\alpha^6+\beta^6).r1​=α4+β4,r2​=101​(α6+β6).

So we must find:

  • r1+r2r_1+r_2r1​+r2​
  • r1r2r_1r_2r1​r2​

  1. Compute α2+β2\alpha^2+\beta^2α2+β2

Using α2+β2=(α+β)2−2αβ,\alpha^2+\beta^2=(\alpha+\beta)^2-2\alpha\beta,α2+β2=(α+β)2−2αβ, we get α2+β2=(−22)2−2(−1)=8+2=10.\alpha^2+\beta^2=(-2\sqrt{2})^2-2(-1)=8+2=10.α2+β2=(−22​)2−2(−1)=8+2=10.


  1. Compute α4+β4\alpha^4+\beta^4α4+β4

Use α4+β4=(α2+β2)2−2α2β2.\alpha^4+\beta^4=(\alpha^2+\beta^2)^2-2\alpha^2\beta^2.α4+β4=(α2+β2)2−2α2β2. Since α2β2=(αβ)2=1,\alpha^2\beta^2=(\alpha\beta)^2=1,α2β2=(αβ)2=1, we get α4+β4=102−2(1)=100−2=98.\alpha^4+\beta^4=10^2-2(1)=100-2=98.α4+β4=102−2(1)=100−2=98.

Thus, r1=98.r_1=98.r1​=98.


  1. Compute α3+β3\alpha^3+\beta^3α3+β3

Use α3+β3=(α+β)3−3αβ(α+β).\alpha^3+\beta^3=(\alpha+\beta)^3-3\alpha\beta(\alpha+\beta).α3+β3=(α+β)3−3αβ(α+β). So, α3+β3=(−22)3−3(−1)(−22).\alpha^3+\beta^3=(-2\sqrt{2})^3-3(-1)(-2\sqrt{2}).α3+β3=(−22​)3−3(−1)(−22​). Now, (−22)3=−162,(-2\sqrt{2})^3=-16\sqrt{2},(−22​)3=−162​, therefore α3+β3=−162−62=−222.\alpha^3+\beta^3=-16\sqrt{2}-6\sqrt{2}=-22\sqrt{2}.α3+β3=−162​−62​=−222​.


  1. Compute α6+β6\alpha^6+\beta^6α6+β6

Use α6+β6=(α3+β3)2−2α3β3.\alpha^6+\beta^6=(\alpha^3+\beta^3)^2-2\alpha^3\beta^3.α6+β6=(α3+β3)2−2α3β3. Now, α3β3=(αβ)3=(−1)3=−1.\alpha^3\beta^3=(\alpha\beta)^3=(-1)^3=-1.α3β3=(αβ)3=(−1)3=−1. Hence, α6+β6=(−222)2−2(−1).\alpha^6+\beta^6=(-22\sqrt{2})^2-2(-1).α6+β6=(−222​)2−2(−1). Since (−222)2=484⋅2=968,(-22\sqrt{2})^2=484\cdot 2=968,(−222​)2=484⋅2=968, we get α6+β6=968+2=970.\alpha^6+\beta^6=968+2=970.α6+β6=968+2=970.

Thus, r2=110(α6+β6)=97010=97.r_2=\frac{1}{10}(\alpha^6+\beta^6)=\frac{970}{10}=97.r2​=101​(α6+β6)=10970​=97.


  1. Form the required quadratic

The required roots are 989898 and 979797.

So the quadratic equation is x2−(98+97)x+(98)(97)=0.x^2-(98+97)x+(98)(97)=0.x2−(98+97)x+(98)(97)=0.

Now, 98+97=195,98+97=195,98+97=195, and 98⋅97=9506.98\cdot 97=9506.98⋅97=9506.

Therefore the equation is x2−195x+9506=0.x^2-195x+9506=0.x2−195x+9506=0.


  1. Check with options

This matches: Option B: x2−195x+9506=0x^2-195x+9506=0x2−195x+9506=0.


  1. Comparison with stored answer

Stored correct answer: B

Our derived answer: B

So they agree.

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