You may have found $\displaystyle{{\sin}}^{{-{1}}}{\left(\frac{{1}}{{7}}\right)}\usepackage{cmbright}\Huge\$ , $\displaystyle{{\tan}}^{{-{1}}}{\left(\frac{{1}}{{7}}\right)}\usepackage{cmbright}\Huge\$ , or just $\displaystyle\frac{{1}}{{7}}\usepackage{cmbright}\Huge\$ . You need to find $\displaystyle{{\cos}}^{{-{1}}}{\left(\frac{{1}}{{7}}\right)}\usepackage{cmbright}\Huge\$ . You also may not have had your calculator set in degree mode.