Other quadrilaterals and polygons

Back to basics: this unit is about the basic stuff that everyone supposedly knows:

Do you know the area formulas for all common quadrilaterals?
Can you easily work out the angles of arbitrarily large regular polygons?
How about counting their diagonals?

Examples

Example

What’s the measurement of the interior angle of a $12$ -gon?

(spoiler)

If you used the formula, then you get full credit. Good enough.

But if you also wanted style points for cleverness, then you divided the sum of the exterior angles (always $36 0^{o}$ ) by $12$ to get that the exterior angles measure $3 0^{o}$ each, and thus the interior angles measure $15 0^{o}$ each.