With exponentiation, there is a difficulty to be overcome. There is no real number that gives −1 when it is squared. In other words, −1 (or any other negative number) has no real square root. For example, there is no real number formula_6 that solves the equation formula_7. To solve this problem, mathematicians introduced a symbol "i" and called it the "imaginary unit". This is the imaginary number that will give −1 when it is squared. 