JEE Challenger
More from Complex Numbers and Quadratic Equations

Construct Quadratic Equation from Ratio of Symmetric Complex Roots

Let p\mathrm{p} and q\mathrm{q} be real numbers such that p≠0\mathrm{p} \neq 0, p3≠q\mathrm{p}^3 \neq \mathrm{q} and p3≠−q\mathrm{p}^3 \neq -\mathrm{q}. If α\alpha and β\beta are nonzero complex numbers satisfying α+β=−p\alpha + \beta = -\mathrm{p} and α3+β3=q\alpha^3 + \beta^3 = \mathrm{q}, then a quadratic equation having αβ\frac{\alpha}{\beta} and βα\frac{\beta}{\alpha} as its roots is

Options

A

(p3+q)x2−(p3+2q)x+(p3+q)=0(\mathrm{p}^3 + \mathrm{q})\mathrm{x}^2 - (\mathrm{p}^3 + 2\mathrm{q})\mathrm{x} + (\mathrm{p}^3 + \mathrm{q}) = 0

B

(p3+q)x2−(p3−2q)x+(p3+q)=0(\mathrm{p}^3 + \mathrm{q})\mathrm{x}^2 - (\mathrm{p}^3 - 2\mathrm{q})\mathrm{x} + (\mathrm{p}^3 + \mathrm{q}) = 0

Correct
C

(p3−q)x2−(5p3−2q)x+(p3−q)=0(\mathrm{p}^3 - \mathrm{q})\mathrm{x}^2 - (5\mathrm{p}^3 - 2\mathrm{q})\mathrm{x} + (\mathrm{p}^3 - \mathrm{q}) = 0

D

(p3−q)x2−(5p3+2q)x+(p3−q)=0(\mathrm{p}^3 - \mathrm{q})\mathrm{x}^2 - (5\mathrm{p}^3 + 2\mathrm{q})\mathrm{x} + (\mathrm{p}^3 - \mathrm{q}) = 0

Step-by-Step Solution

To find a quadratic equation whose roots are αβ\frac{\alpha}{\beta} and βα\frac{\beta}{\alpha}, we first determine the sum and the product of these roots.

Let the roots be r1=αβr_1 = \frac{\alpha}{\beta} and r2=βαr_2 = \frac{\beta}{\alpha}.

The sum of the roots is:
S=r1+r2=αβ+βα=α2+β2αβS = r_1 + r_2 = \frac{\alpha}{\beta} + \frac{\beta}{\alpha} = \frac{\alpha^2 + \beta^2}{\alpha\beta}

The product of the roots is:
P=r1⋅r2=αβ⋅βα=1P = r_1 \cdot r_2 = \frac{\alpha}{\beta} \cdot \frac{\beta}{\alpha} = 1

We are given:
α+β=−p\alpha + \beta = -p
α3+β3=q\alpha^3 + \beta^3 = q

Using the algebraic identity for the sum of cubes:
α3+β3=(α+β)3−3αβ(α+β)\alpha^3 + \beta^3 = (\alpha + \beta)^3 - 3\alpha\beta(\alpha + \beta)

Substitute the given values into the identity:
q=(−p)3−3αβ(−p)q = (-p)^3 - 3\alpha\beta(-p)
q=−p3+3pαβq = -p^3 + 3p\alpha\beta

Since p≠0p \neq 0, we can solve for the product αβ\alpha\beta:
3pαβ=p3+q  ⟹  αβ=p3+q3p3p\alpha\beta = p^3 + q \implies \alpha\beta = \frac{p^3 + q}{3p}

Next, we express α2+β2\alpha^2 + \beta^2 in terms of pp and qq:
α2+β2=(α+β)2−2αβ\alpha^2 + \beta^2 = (\alpha + \beta)^2 - 2\alpha\beta
α2+β2=(−p)2−2(p3+q3p)=p2−2(p3+q)3p\alpha^2 + \beta^2 = (-p)^2 - 2\left(\frac{p^3 + q}{3p}\right) = p^2 - \frac{2(p^3 + q)}{3p}
α2+β2=3p3−2p3−2q3p=p3−2q3p\alpha^2 + \beta^2 = \frac{3p^3 - 2p^3 - 2q}{3p} = \frac{p^3 - 2q}{3p}

Now, substitute α2+β2\alpha^2 + \beta^2 and αβ\alpha\beta into the expression for SS:
S=α2+β2αβ=p3−2q3pp3+q3p=p3−2qp3+qS = \frac{\alpha^2 + \beta^2}{\alpha\beta} = \frac{\frac{p^3 - 2q}{3p}}{\frac{p^3 + q}{3p}} = \frac{p^3 - 2q}{p^3 + q}

The quadratic equation with roots r1r_1 and r2r_2 is given by:
x2−Sx+P=0x^2 - Sx + P = 0
x2−(p3−2qp3+q)x+1=0x^2 - \left(\frac{p^3 - 2q}{p^3 + q}\right)x + 1 = 0

Since p3≠−qp^3 \neq -q, we can multiply the entire equation by (p3+q)(p^3 + q):
(p3+q)x2−(p3−2q)x+(p3+q)=0(p^3 + q)x^2 - (p^3 - 2q)x + (p^3 + q) = 0

Therefore, the correct option is B.