You may have been thinking about the fact that the symbol $\displaystyle\sqrt{\ \text{ }\ }\usepackage{cmbright}\Huge\$ is defined to be the positive square root of a number. Because $\displaystyle-{1}=-{1}\cdot-{1}\cdot-{1}\usepackage{cmbright}\Huge\$ , the cube root of $\displaystyle-{1}\usepackage{cmbright}\Huge\$ is $\displaystyle-{1}\usepackage{cmbright}\Huge\$ . This is a negative number.