A negative exponent does not mean that the number is negative. Since $\displaystyle{{n}}^{{-{x}}}=\frac{{1}}{{{n}}^{{x}}}\usepackage{cmbright}\Huge\$ , this tells you that $\displaystyle{{5}}^{{-{{1}}}}=\frac{{1}}{{{5}}^{{1}}}\usepackage{cmbright}\Huge\$