Filled With Circles 2

Geometry Level pending

X Y Z XYZ is an isosceles triangle with X Y = 4 XY = 4 .

A , B , C , D , A,B,C,D,\ldots are infinitely many circles tangent to the triangle and to their neighboring circles.

Find the sum of areas of all these circles to 3 decimal places.


The answer is 12.19117.

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2 solutions

Marta Reece
Apr 14, 2017

2 × C Y O + C Z Y = 9 0 2\times\angle CYO+\angle CZY=90^\circ

C Y O = 1 2 × ( 9 0 a r c t a n ( 1 4 ) ) = 37.9818 8 \angle CYO=\frac{1}{2}\times (90^\circ-arctan(\frac{1}{4}))=37.98188^\circ

r = 2 × t a n ( C Y O ) = 1.56155 r=2\times tan(\angle CYO)=1.56155

b = 8 2 r , a b = 2 8 , a = 8 2 r 4 = 1.21922 b=8-2r, \frac{a}{b}=\frac{2}{8}, a=\frac{8-2r}{4}=1.21922

Area of trapezoid A B Y X ABYX is A t = 1 2 ( 4 + 2 a ) 2 R = 10.0539 A_t=\frac{1}{2}(4+2a)2R=10.0539

Area of the circle centered at O is A c = π r 2 = 7.6606 A_c=\pi r^2=7.6606

The ratio of the areas A c A t = 0.761948 \frac{A_c}{A_t}=0.761948

This ratio remains the same for all the circles below as well, each within its own trapezoid, so the total area of circles is this ratio times the total area of X Y Z \triangle XYZ

A = 0.761948 × 1 2 × 4 × 8 = 12.19117 A=0.761948\times\frac{1}{2}\times 4\times 8=12.19117

Guiseppi Butel
Apr 15, 2017

Using the method of finding the radius in "Filled with Circles" calculate the areas of the 1st 2 circles.

Then find the ratio Area(B)/Area(A) and use these figures to calculate the sum of an infinite geometric series.

Sum = Area(A)/(1- r)

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