$\sqrt -20=i\sqrt 20 = i\sqrt (4 \times 5) = 2i\sqrt 5$
Work Step by Step
Using $\sqrt -b = i\sqrt b$ $\sqrt -20=i\sqrt 20$ $ = i\sqrt (4 \times 5) $ $=i \sqrt 4 \times \sqrt 5$ $= 2i\sqrt 5$
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