Answer
$\log_{\frac{1}{2}}{4}=-2$
Work Step by Step
RECALL:
$y=a^b \longleftrightarrow \log_a{y}=b$
Use the definition above to have
$4 = \left(\frac{1}{2}\right)^{-2} \longleftrightarrow \log_{\frac{1}{2}}{4}=-2.$
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