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Roots of Quadratic Equations forming a Geometric Progression

Let α,β\alpha, \beta be the roots of the equation x2x+p=0x^2 - x + p = 0 and γ,δ\gamma, \delta be the roots the equation x24x+q=0x^2 - 4x + q = 0; p,qZp, q \in \mathbb{Z}. If α,β,γ,δ\alpha, \beta, \gamma, \delta are in G.P., then p+q|p + q| equals :

Options

A

16

B

32

C

34

Correct
D

38

Step-by-Step Solution

Given that α,β\alpha, \beta are the roots of the quadratic equation x2x+p=0x^2 - x + p = 0 and γ,δ\gamma, \delta are the roots of x24x+q=0x^2 - 4x + q = 0, where p,qZp, q \in \mathbb{Z}.

By Vieta's formulas, we have: α+β=1andαβ=p\alpha + \beta = 1 \quad \text{and} \quad \alpha \beta = p γ+δ=4andγδ=q\gamma + \delta = 4 \quad \text{and} \quad \gamma \delta = q

Since α,β,γ,δ\alpha, \beta, \gamma, \delta are in Geometric Progression (G.P.), let the terms be represented as: α=a,β=ar,γ=ar2,δ=ar3\alpha = a, \quad \beta = ar, \quad \gamma = ar^2, \quad \delta = ar^3 where aa is the first term and rr is the common ratio.

Substituting these terms into the equations for the sum of roots: α+β=a+ar=a(1+r)=1— (1)\alpha + \beta = a + ar = a(1 + r) = 1 \quad \text{--- (1)} γ+δ=ar2+ar3=ar2(1+r)=4— (2)\gamma + \delta = ar^2 + ar^3 = ar^2(1 + r) = 4 \quad \text{--- (2)}

Dividing equation (2) by equation (1): ar2(1+r)a(1+r)=41\frac{ar^2(1 + r)}{a(1 + r)} = \frac{4}{1} r2=4    r=±2r^2 = 4 \implies r = \pm 2

Now, we evaluate the two possible cases for rr using the condition p,qZp, q \in \mathbb{Z}:

Case 1: r=2r = 2 From equation (1): a(1+2)=1    a=13a(1 + 2) = 1 \implies a = \frac{1}{3}

Using the product of roots: p=αβ=a2r=(13)2(2)=29p = \alpha \beta = a^2 r = \left(\frac{1}{3}\right)^2 (2) = \frac{2}{9} Since p=29Zp = \frac{2}{9} \notin \mathbb{Z}, this case is rejected.

Case 2: r=2r = -2 From equation (1): a(12)=1    a=1a(1 - 2) = 1 \implies a = -1

Using the products of roots: p=αβ=a2r=(1)2(2)=2Zp = \alpha \beta = a^2 r = (-1)^2 (-2) = -2 \in \mathbb{Z} q=γδ=a2r5=(1)2(2)5=32Zq = \gamma \delta = a^2 r^5 = (-1)^2 (-2)^5 = -32 \in \mathbb{Z}

Since both pp and qq are integers, r=2r = -2 is the valid ratio.

Now, calculating p+q|p + q|: p+q=(2)+(32)=34=34|p + q| = |(-2) + (-32)| = |-34| = 34

Hence, the correct option is C.

Roots of Quadratic Equations forming a Geometric Progression | Mathematics PYQ Solution - JEE Challenger