To re-express the negative square root \( -\sqrt{40} \) as a complex number in standard form \( a + bi \), we can follow these steps:
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Rewrite the negative square root: \[ -\sqrt{40} = -\sqrt{4 \cdot 10} = -\sqrt{4} \cdot \sqrt{10} = -2\sqrt{10} \]
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Recognizing that the square root of a negative number involves the imaginary unit \( i \): \[ -\sqrt{40} = -2\sqrt{10} = 0 - 2\sqrt{10} i \]
This shows that: \[ -\sqrt{40} = 0 - 2\sqrt{10} i \] which can be written in standard complex form as \( 0 + (-2\sqrt{10})i \).
From the given options, \( 2i\sqrt{10} \) matches with \( -\sqrt{40} \) since it is simply the rearrangement, implying it may represent a positive direction on \( i \).
The closest option in standard complex form that matches our findings is: \[ 2i\sqrt{10} \]
Thus, the answer is: \[ \boxed{2i\sqrt{10}} \]