9514 1404 393
Answer:
i
Explanation:
The square root of -1 is the number that, when multiplied by itself, gives the product -1. It does not have a numerical value in the same way that the root of a positive number has a numerical value. It can only be represented by a symbol of some sort: √(-1) or i.
i² = -1
In some fields, the letter j is used instead (because i already stands for something different).
i = √(-1)
j = √(-1) . . . in electrical engineering (at least)