You may have chosen this approximation because it falls between 9 and 10 and you know that $\displaystyle\sqrt{{{88}}}\usepackage{cmbright}\Huge\$ is between $\displaystyle\sqrt{{{81}}}\usepackage{cmbright}\Huge\$ and $\displaystyle\sqrt{{{100}}}\usepackage{cmbright}\Huge\$ . However, because 88 is closer to 81 than it is to 100, the approximation will be closer to 9 than to 10. Using trial and error, square different approximations to find the best estimate of $\displaystyle\sqrt{{{88}}}\usepackage{cmbright}\Huge\$ .