AMC 10 Daily Practice - Polygons

Complete problem set with solutions and individual problem pages

Problem 2 Easy

Given that the distance from the center of a regular dodecagon (12-sided polygon) to one of its vertices is 45, and the internal blank area is composed of squares and triangles, the area of the shaded region is            .

  • A.

    2000

  • B.

    2010

  • C.

    2015

  • D.

    2025

  • E.

    2030

Answer:D

Solution 1:

r=45^\circ

x^2 = 45^2 + 45^2 - 2 \cdot 45 \cdot 45 -\frac{\sqrt3}{2} = 2 \cdot 45^2 - 45^2\cdot \sqrt{3}

A = 12 \cdot \frac{1}{2} \cdot 45 \cdot 45 \cdot \frac{1}{2}

= 3 \cdot 45^2

A_\triangle = \frac{\sqrt{3}}{4} \cdot x^2 = \frac{\sqrt{3}}{4} (2 \cdot 45^2 - 45^2\cdot \sqrt{3})

8A_\triangle = 2\sqrt{3} (2 \cdot 45^2 - 45^2\cdot \sqrt{3})

4A_\square = 8 \cdot 45^2 - 4\cdot 45^2\sqrt{3}

8A_\triangle = 4\sqrt{3} \cdot 45^2 - 6 \cdot 45^2

4A_\square + 8A_\triangle = 2 \cdot 45^2

A = 45^2 = 2025

[Note: x represent the side length of dodecagon]

Solution 2:

(x + \frac{\sqrt{3}}{2}x)^2 + (\frac{x}{2})^2 = 45^2

x^2 + \frac{3}{4}x^2 + \sqrt{3}x^2 + \frac{x^2}{4} = 45^2

(2+\sqrt{3})x^2 = 45^2

x^2 = \frac{45^2}{2 + \sqrt{3}}

\frac{1}{2}x(\frac{\sqrt{3}}{2}x+x) = \frac{\sqrt{3} + 2}{4} x^2

A = 12 \cdot \frac{\sqrt{3} + 2}{4}x^2

= 3(\sqrt{3} + 2) \cdot \frac{45^2}{2 + \sqrt{3}}

= 3 \cdot 45^2

A_\triangle = \frac{\sqrt{3}}{4}x^2 \cdot 8

= \frac{\sqrt{3}}{4} \cdot \frac{45^2}{2 + \sqrt{3}} \cdot 8

= \frac{2\sqrt{3} \cdot 45^2}{2 + \sqrt{3}}

A_\square = x^2 \cdot 4

= \frac{4 \cdot 45^2}{2 + \sqrt{3}}

A_{\triangle + \square} = \frac{2\sqrt{3} \cdot 45^2 + 4 \cdot 45^2}{2 + \sqrt{3}}

= \frac{2 \cdot 45^2 \cdot (\sqrt{3} + 2)}{2 + \sqrt{3}}

= 2 \cdot 45^2

A_{shaded} = 3 \cdot 45^2 - 2\cdot 45^2

= 45^2

= 2023

[Note: x represent the side length of dodecagon]