What is the sum of the interior angles of a hendecagon?

Sum of interior angles of a hendecagon ##=1620^@##

The real issue here is knowing that a "hendecagon" is an ##11## sided (and angled) polygon.

The formula for the sum of the interior angles of an ##n##-gon is ##color(white)("XXX")180^@xx(n-2)color(white)("XXXXX")##provided ##n>=3##

An ##11##-gon (hendecagon) can be divided into ##9## triangles by connecting vertices; each triangle has an interior angle sum of ##180^@##

Sum of interior angles of hendecagon ##=180^@ xx 9 = 1620^@##