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MathNet 2010 — Part 05 Question 22

Math Olympiad Vault · MathNet 2010 — Part 05

Question 22

Consider $v, w$ two distinct non-zero complex numbers. Prove that
$$
|zw + \bar{w}| \leq |zv + \bar{v}|,
$$
for any $z \in \mathbb{C}$, $|z| = 1$, if and only if there exists $k \in [-1, 1]$ such that $w = kv$.