We know that square of a real number is always non-negative e.g.  and  . Therefore, square root of  is . What about the square root of a negative number? It is clear that a negative number can not have a real square root. So we need to extend the system of real numbers to a system in which we can find out the square roots of negative numbers. Euler (1707-1783) was the first mathematician to introduce the symbol (iota) for positive square root of  i.e. , .