Marris decides to bake his parents a cookie in the shape of a regular dodecagon (12‑gon) for their 12-day anniversary.

A.) If the edge of the dodecagon is 6 cm, what is the area of the top of the cookie?

B.) His parents decides to divide the cookie into 12 congruent pieces.
After 9 of the pieces have been eaten, what area of the cookie is left?

In the exercise, we have the edge (6 cm), but, to find the apothem of a dodecagon, that is the distance between the center of the polygon, and the middle point of each side, you can use the next formula:

Apothem = Edge * [tex]\frac{2+\sqrt{3}}{2}[/tex]

If we replace the value of the edge, we obtain:

Apothem = 6 cm * [tex]\frac{2+\sqrt{3}}{2}[/tex]
Apothem = 11.2 cm (approximately).

With this data, we can find the area:

Area of a dodecagon = 6 * apothem * edge
Area of a dodecagon = 6 * 11.2 cm * 6 cm
Area of a dodecagon = 403.2 [tex]cm^{2}[/tex]

Then, the area of the dodecagon is 403.2 [tex]cm^{2}[/tex] approximately.

B.) As the area obtained is for 12 pieces, when the parents ate 9 pieces, just left 3 pieces, with these values you can make a rule of three.

If:

12 pieces = 403.2 [tex]cm^{2}[/tex]
3 pieces = X

Then:

X = (3 * 403.2 [tex]cm^{2}[/tex]) / 12
X = 1209.6 [tex]cm^{2}[/tex] / 12
X = 100. 8 [tex]cm^{2}[/tex]

In this form, when the cookie has 3 pieces, its area is 100. 8 [tex]cm^{2}[/tex] approximately.