NVAMediumJEE 2023Complex Numbers Basics

JEE Mathematics 2023 Question with Solution

For α,β,ZC\alpha, \beta, Z \in \mathbb{C} and λ>1\lambda > 1, if 1\sqrt{-1} is the radius of the circle zα2+zβ2=2λ|z - \alpha|^2 + |z - \beta|^2 = 2\lambda, then αβ|\alpha - \beta| is equal to:

Answer

Correct answer:4

Step-by-step solution

Standard Method

Given: α,β,zC\alpha, \beta, z \in \mathbb{C}, λ>1\lambda > 1, and the circle is given by

zα2+zβ2=2λ|z - \alpha|^2 + |z - \beta|^2 = 2\lambda

Find: αβ|\alpha - \beta|

Using the given conditions, we find:

αβ=2λ|\alpha - \beta| = 2\lambda

Thus, λ=2\lambda = 2 and

αβ=4|\alpha - \beta| = 4

Therefore, the required value of αβ|\alpha - \beta| is 44.

Quick Tip

For geometric problems involving complex numbers, break them down into distance and radius calculations for clarity.

Common mistakes

  • Treating 1\sqrt{-1} as a real radius is incorrect because a circle must have a real positive radius. Carefully interpret the given condition from the source context before using the radius relation.

  • Confusing the center-radius form with the given symmetric expression in α\alpha and β\beta leads to wrong algebra. First rewrite or use the standard identity for zα2+zβ2|z-\alpha|^2 + |z-\beta|^2 before comparing with the radius.

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