A square root of a number a is a number y such that x2 = a, in other words, a number x whose Square and 2 is Power of x.
81 = 9 x 9 = (-9) x (-9), where 9 and (-9) are square roots of 81.
Every positive real numbers x has a unique non-negative square root, called the principal square root, which is denoted by √x, where
√ is called the radix.
For example, the principal square root of 81 is 9, denoted √81 = 9, because 92 = 9 × 9 = 81 and 9 is non-negative.
Every positive number x has two square roots: √x, which is positive, and −√x, which is negative. Together, these two roots are denoted ± √a.