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[复变函数] (171023) 设 $z_1,\cdots,z_n$ 是复数, 试证: $$\bex \sex{\sum_{k=1}^n |z_k|^2}^2 -\sev{\sum_{k=1}^n z_k}^2 \geq \sex{\sum_{k=1}^n |\Re z_k| -\sev{\sum_{k=1}^n \Re z_k}}^2. \eex$$ |
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