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Question

Let \(Z_1\) and \(Z_2\) be complex numbers such that \(\frac{3Z_1}{4Z_2}\) is purely imaginary.
What is \(\left| \frac{Z_1 + Z_2}{Z_1 - Z_2} \right|\) equal to ?

This question was previously asked in
NDA 2 2026 GAT Question Paper (13-Sep-2026)
The correct answer is
1

Complex Number Magnitude Calculation

The problem asks for the value of \(\left| \frac{Z_1 + Z_2}{Z_1 - Z_2} \right|\) given that \(\frac{3Z_1}{4Z_2}\) is purely imaginary.

Condition Analysis

A complex number \(w\) is purely imaginary if its real part is zero, which implies \(w = - \bar{w}\) (for \(w \neq 0\)).

Given that \(\frac{3Z_1}{4Z_2}\) is purely imaginary:

\( \frac{3Z_1}{4Z_2} = - \overline{\left(\frac{3Z_1}{4Z_2}\right)} \) \( \frac{3Z_1}{4Z_2} = - \frac{3\bar{Z_1}}{4\bar{Z_2}} \)

Simplifying this equation by canceling out the common terms \(\frac{3}{4}\):

\( \frac{Z_1}{Z_2} = - \frac{\bar{Z_1}}{\bar{Z_2}} \)

This can be rewritten as:

\( \frac{Z_1}{Z_2} = - \overline{\left(\frac{Z_1}{Z_2}\right)} \)

This shows that the ratio \(\frac{Z_1}{Z_2}\) is itself a purely imaginary number. Let's represent this ratio as \(ik\), where \(k\) is a non-zero real number.

\( \frac{Z_1}{Z_2} = ik, \quad k \in \mathbb{R}, k \neq 0 \)

Magnitude Calculation

Now, we need to find the magnitude of the expression \(\frac{Z_1 + Z_2}{Z_1 - Z_2}\). Divide both the numerator and the denominator by \(Z_2\) (assuming \(Z_2 \neq 0\)):

\( \frac{Z_1 + Z_2}{Z_1 - Z_2} = \frac{\frac{Z_1}{Z_2} + \frac{Z_2}{Z_2}}{\frac{Z_1}{Z_2} - \frac{Z_2}{Z_2}} = \frac{\frac{Z_1}{Z_2} + 1}{\frac{Z_1}{Z_2} - 1} \)

Substitute \(\frac{Z_1}{Z_2} = ik\) into the expression:

\( \frac{ik + 1}{ik - 1} = \frac{1 + ik}{-1 + ik} \)

Finally, calculate the magnitude of this complex number:

\( \left| \frac{1 + ik}{-1 + ik} \right| = \frac{|1 + ik|}{|-1 + ik|} \)

The magnitude of a complex number \(a+bi\) is \(\sqrt{a^2 + b^2}\). Therefore:

\( \frac{|1 + ik|}{|-1 + ik|} = \frac{\sqrt{1^2 + k^2}}{\sqrt{(-1)^2 + k^2}} = \frac{\sqrt{1 + k^2}}{\sqrt{1 + k^2}} \) \( \frac{\sqrt{1 + k^2}}{\sqrt{1 + k^2}} = 1 \)

Thus, the magnitude \(\left| \frac{Z_1 + Z_2}{Z_1 - Z_2} \right|\) is equal to 1.

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Important Questions from Complex Numbers

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