If \( |z| = 5 \) and \( w = \frac{z-5}{z+5} \), then \( \text{Re}(w) \) is equal to:
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The mapping \( w = \frac{z-k}{z+k} \) maps the circle \( |z|=k \) onto the imaginary axis (where \( \text{Re}(w)=0 \)). This is a fundamental property of Möbius transformations.