The area of the yellow region in the regular five-pointed star shown below is
Find the area of the blue region rounded to the nearest integer.
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The general area of a regular n-sided polygon with side length S is given as ∣ A r e a ∣ = 4 n S 2 C o t ( n π ) The Area of the Yellow Pentagon is given by 4 5 S 2 C o t ( 5 π ) = 5 0 0
Each Blue Triangle is isosceles and has a base length of S. The Internal Angle of the Pentagon is given by n ( n − 2 ) π = 5 3 π so the base angle of the triangle is π − 5 3 π = 5 2 π
The Area of each Blue Triangle is given by ( 2 S ) ( 2 S ) T a n ( 5 2 π ) = 4 S 2 T a n ( 5 2 π )
There are 5 Blue Triangles so the area of the Blue Region is given by ∣ B l u e ∣ = 4 5 S 2 T a n ( 5 2 π )
Dividing the two equations we have 5 0 0 ∣ B l u e ∣ = 4 5 S 2 C o t ( 5 π ) 4 5 S 2 T a n ( 5 2 π )
Rearranging yields ∣ B l u e ∣ = 5 0 0 T a n ( 5 π ) T a n ( 5 2 π )
There doesn't seem to be a nice identity simplifying T a n ( x ) T a n ( 2 x ) so the answer is simply calculated as 1 1 1 8
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})\\