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GATE 2025 MA – Question 41

Complex Analysis · Conformal Mappings · 2 marks · Multiple choice

A Möbius transformation sends 0,1/2,1 to −3,∞,2. It maps the circle centered at 1 with radius k to line αx+βy+γ=0. Find [2k(α+β)+γ]/[α+β−2kγ].

  1. 1/7
  2. 2/7
  3. 1/3
  4. 2/3

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Correct answer: (A) 1/7

Explanation

The map is $w=(3-z)/(2z-1)$. A circle maps to a line iff it contains the pole 1/2, so k=1/2. Invert to z=(w+3)/(2w+1); |z−1|=1/2 becomes 4|w−2|²=|2w+1|², simplifying to 4x−3=0. Choose α=4,β=0,γ=−3. Required ratio=(4−3)/(4+3)=**1/7, A**.

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