The square root of a negative number such as -4 is a complex number, and cannot be represented on the real number line. In complex analysis, the square root of -1 is represented by the imaginary unit i, which is defined as i = √(-1). Therefore, the square root of -4 can be written as:
√(-4) = √(4) * √(-1) = 2i
So the square root of -4 is 2i, where i is the imaginary unit. This result is used in many mathematical and engineering applications, including electrical engineering, signal processing, and quantum mechanics.