You realized that i is used in the square root of a negative number. However, you forgot that the negative sign was already used in the definition of i, and inserted an extra one. Use the rule $\displaystyle\sqrt{{{a}{b}}}=\sqrt{{{a}}}\sqrt{{{b}}}\usepackage{cmbright}\Huge\$ and the definition $\displaystyle{i}=\sqrt{{-{1}}}\usepackage{cmbright}\Huge\$ to simplify.