Step-by-step explanation:
there is an official formula for this. this can be truly found very easily on the internet ...
the formula for the sum of all angles in a regular polygon with n sides or vertices is
(n-2)×180
so, since an octagon has 8 sides and vertices, this gives us
(8-2)×180 = 6 × 180 = 1080°
that's it ...
