The number k is a square root of the number n if k2 = n. For example, 4 and -4 are the square roots of 16 since 4 x 4 = 16 and (-4) x (-4) = 16. The square root symbol is and for n greater than zero the symbol is understood to be a positive number. Thus 16 = 4 and -16 = -4.

When n is a negative number, the square root n is called imaginary. Customarily, -1 is designated by i so that the square root of any negative number can be expressed as ai where a is a real number. Thus -5 = 5 i.