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Quadratic Equation and Inequalities question

2020 · 4 Sep · Shift 1 · Q30
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  5. /2020 · 4 Sep · Shift 1 · Q30

Quadratic Equation and Inequalities question

2020 · 4 Sep · Shift 1 · Q30

JEE MainMathematicsQuadratic Equation and InequalitiesMCQ+4 / −1
Let α\alphaα and β\betaβ be the roots of x2 - 3x + p=0 and γ\gammaγ and δ\deltaδ be the roots of x2 - 6x + q = 0. If α,β,γ,δ\alpha, \beta, \gamma, \deltaα,β,γ,δ form a geometric progression.Then ratio (2q + p) : (2q - p) is:
  1. A
    9 : 7
  2. B
    5 : 3
  3. C
    3 : 1
  4. D
    33 :31
View written solutionFree

Correct answer: A

  1. Use Vieta's formulas

For the quadratic x2−3x+p=0,x^2-3x+p=0,x2−3x+p=0, with roots α,β\alpha,\betaα,β: α+β=3,αβ=p.\alpha+\beta=3, \qquad \alpha\beta=p.α+β=3,αβ=p.

For the quadratic x2−6x+q=0,x^2-6x+q=0,x2−6x+q=0, with roots γ,δ\gamma,\deltaγ,δ: γ+δ=6,γδ=q.\gamma+\delta=6, \qquad \gamma\delta=q.γ+δ=6,γδ=q.


  1. Use the condition that α,β,γ,δ\alpha,\beta,\gamma,\deltaα,β,γ,δ are in geometric progression

Let the four numbers in GP be a,ar,ar2,ar3.a, ar, ar^2, ar^3.a,ar,ar2,ar3. Then we may take α=a,β=ar,γ=ar2,δ=ar3.\alpha=a,\quad \beta=ar,\quad \gamma=ar^2,\quad \delta=ar^3.α=a,β=ar,γ=ar2,δ=ar3.

So, α+β=a(1+r)=3⋯(1)\alpha+\beta=a(1+r)=3 \quad \cdots (1)α+β=a(1+r)=3⋯(1) γ+δ=ar2(1+r)=6⋯(2)\gamma+\delta=ar^2(1+r)=6 \quad \cdots (2)γ+δ=ar2(1+r)=6⋯(2)

Divide (2) by (1): ar2(1+r)a(1+r)=63\frac{ar^2(1+r)}{a(1+r)}=\frac{6}{3}a(1+r)ar2(1+r)​=36​ r2=2.r^2=2.r2=2.

Hence, r=±2.r=\pm \sqrt{2}.r=±2​. But for our required ratio, only r2=2r^2=2r2=2 is needed.


  1. Find aaa using (1)

From a(1+r)=3,a(1+r)=3,a(1+r)=3, we get a=31+r.a=\frac{3}{1+r}.a=1+r3​.


  1. Compute ppp and qqq

Since p=αβ=a⋅ar=a2r,p=\alpha\beta=a\cdot ar=a^2r,p=αβ=a⋅ar=a2r, we get p=(31+r)2r=9r(1+r)2.p=\left(\frac{3}{1+r}\right)^2 r=\frac{9r}{(1+r)^2}.p=(1+r3​)2r=(1+r)29r​.

Also, q=γδ=(ar2)(ar3)=a2r5,q=\gamma\delta=(ar^2)(ar^3)=a^2r^5,q=γδ=(ar2)(ar3)=a2r5, so q=(31+r)2r5=9r5(1+r)2.q=\left(\frac{3}{1+r}\right)^2 r^5=\frac{9r^5}{(1+r)^2}.q=(1+r3​)2r5=(1+r)29r5​.

Now use r2=2r^2=2r2=2: r4=4⇒r5=4r.r^4=4 \Rightarrow r^5=4r.r4=4⇒r5=4r. Thus, q=9⋅4r(1+r)2=4p.q=\frac{9\cdot 4r}{(1+r)^2}=4p.q=(1+r)29⋅4r​=4p.


  1. Find the required ratio

We need (2q+p):(2q−p).(2q+p):(2q-p).(2q+p):(2q−p). Since q=4pq=4pq=4p, 2q+p=2(4p)+p=9p,2q+p=2(4p)+p=9p,2q+p=2(4p)+p=9p, 2q−p=2(4p)−p=7p.2q-p=2(4p)-p=7p.2q−p=2(4p)−p=7p.

Therefore, (2q+p):(2q−p)=9p:7p=9:7.(2q+p):(2q-p)=9p:7p=9:7.(2q+p):(2q−p)=9p:7p=9:7.


  1. Check options

The correct option is: 9:7\boxed{9:7}9:7​ which is Option A.


  1. Comparison with stored answer

Stored correct answer: A

Our derived answer: A

So they agree.

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