Find the area.

Geometry Level 3

The area of the yellow region in the regular five-pointed star shown below is 500. 500.

Find the area of the blue region rounded to the nearest integer.


The answer is 1118.

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1 solution

Les Schumer
Jul 7, 2020

The general area of a regular n-sided polygon with side length S is given as A r e a = n S 2 4 C o t ( π n ) |Area|=\frac{nS^2}{4}Cot(\frac{\pi}{n})\\ The Area of the Yellow Pentagon is given by 5 S 2 4 C o t ( π 5 ) = 500 \frac{5S^2}{4}Cot(\frac{\pi}{5})=500\\

Each Blue Triangle is isosceles and has a base length of S. The Internal Angle of the Pentagon is given by ( n 2 ) π n = 3 π 5 \frac{(n-2)\pi}{n} = \frac{3\pi}{5} so the base angle of the triangle is π 3 π 5 = 2 π 5 \pi-\frac{3\pi}{5}=\frac{2\pi}{5}\\

The Area of each Blue Triangle is given by ( S 2 ) ( S 2 ) T a n ( 2 π 5 ) = S 2 4 T a n ( 2 π 5 ) (\frac{S}{2})(\frac{S}{2})Tan(\frac{2\pi}{5})=\frac{S^2}{4}Tan(\frac{2\pi}{5})\\

There are 5 Blue Triangles so the area of the Blue Region is given by B l u e = 5 S 2 4 T a n ( 2 π 5 ) |Blue|=\frac{5S^2}{4}Tan(\frac{2\pi}{5})\\

Dividing the two equations we have B l u e 500 = 5 S 2 4 T a n ( 2 π 5 ) 5 S 2 4 C o t ( π 5 ) \frac{|Blue|}{500}=\frac{\frac{5S^2}{4}Tan(\frac{2\pi}{5})}{\frac{5S^2}{4}Cot(\frac{\pi}{5})}\\

Rearranging yields B l u e = 500 T a n ( π 5 ) T a n ( 2 π 5 ) |Blue|=500Tan(\frac{\pi}{5})Tan(\frac{2\pi}{5})\\

There doesn't seem to be a nice identity simplifying T a n ( x ) T a n ( 2 x ) Tan(x)Tan(2x) so the answer is simply calculated as 1118 \boxed{1118}\\

It does, however, lead to a nice general solution for the ratio of the areas of the "Points" to the "Central area" of any regular polygon as \frac{|Points|}{|Centre|} = Tan(\frac{\pi}{n})Tan{\frac{2\pi}{n})\\

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