If a polygon has [tex] n [/tex] sides, the sum of its interior angles is
[tex] (n-2)\times 180^\circ [/tex]
A nonagon (from greek "[tex] \varepsilon\nu\nu\varepsilon\alpha [/tex]", nine and "[tex] \gamma\omega\nu o \nu [/tex]", angle) has nine sides, so [tex]n=9[/tex]
If you plug these value into the formula for the sum of inner angles, you have
[tex] (n-2)\times 180^\circ = (9-2)\times 180^\circ = 7\times 180^\circ = 1260^\circ [/tex]
