On a scientific calculator, first divide 1 by 7, then press the 2nd key and COS. Do this in the reverse order for a graphing calculator.

$\displaystyle{{\cos}}^{{-{1}}}{\left(\frac{{1}}{{7}}\right)}={{\cos}}^{{-{1}}}{0}.{\overline{{{142857}}}}={81.7867893}{\ldots}^{\circ}\usepackage{cmbright}\Huge\$

$\displaystyle\approx{{81.79}}^{\circ}\usepackage{cmbright}\Huge\$