More of Circles and Squares.

Level pending

Extend the above diagram to n n congruent circles where n n is a even positive integer.

In square A B C D ABCD , one of the vertices of square A J P 1 I AJP_{1}I touches E 1 F 1 \overline{E_{1}F_{1}} at P 1 P_{1} and E j F j \overline{E_{j}F_{j}} is tangent to circle C j C_{j} at P j P_{j} for each integer j j , where ( 1 j n ) (1 \leq j \leq n) and the radius of each congruent circle is half the side of the square A J P 1 I AJP_{1}I .

Let A T A_{T} be the area of the shaded green region.

Find the value of n n for which A T A A B C D = 32 2 41 9 \dfrac{A_{T}}{A_{ABCD}} = \dfrac{32\sqrt{2} - 41}{9}


The answer is 2.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

1 solution

Rocco Dalto
Dec 16, 2020

Let n n be a fixed even integer, a a a side of square A B C D ABCD and x x a side of square A J P 1 I AJP_{1}I .

Extending the diagram to even n n congruent circles we obtain:

2 a = 2 x + ( n 1 ) x + x 2 + x 2 4 a = ( 6 + 2 ( 2 n 1 ) ) x \sqrt{2}a = \sqrt{2}x + (n - 1)x + \dfrac{x}{2} + \dfrac{x}{\sqrt{2}} \implies 4a = (6 + \sqrt{2}(2n - 1))x \implies

x = 4 a 6 + 2 ( 2 n 1 ) = 4 a 2 ( 3 2 + ( 2 n 1 ) ) = x = \dfrac{4a}{6 + \sqrt{2}(2n - 1)} = \dfrac{4a}{\sqrt{2}(3\sqrt{2} + (2n - 1))} = 2 2 3 2 + ( 2 n 1 ) a \dfrac{2\sqrt{2}}{3\sqrt{2} + (2n - 1)}a

C P n 2 + 1 = ( n ( n 2 + 1 ) + 1 2 + 1 2 ) x = \overline{CP_{\frac{n}{2} + 1}} = (n - (\dfrac{n}{2} + 1) + \dfrac{1}{2} + \dfrac{1}{\sqrt{2}})x = ( 2 n + 2 2 2 2 ) ( 2 2 2 n + 3 2 1 ) a = (\dfrac{\sqrt{2}n + 2 - \sqrt{2}}{2\sqrt{2}})(\dfrac{2\sqrt{2}}{2n + 3\sqrt{2} - 1})a =

2 ( n + 2 1 ) 2 n + 3 2 1 a \dfrac{\sqrt{2}(n + \sqrt{2} - 1)}{2n + 3\sqrt{2} - 1}a \implies A 1 = ( C P n 2 + 1 ) 2 = A_{\triangle{1}} = (\overline{CP_{\frac{n}{2} + 1}})^2 = 2 ( n + 2 1 ) 2 ( 2 n + 3 2 1 ) 2 a 2 \dfrac{2(n + \sqrt{2} - 1)^2}{(2n + 3\sqrt{2} - 1)^2}a^2

A P n 2 = ( 2 + ( n 2 1 ) ) x = ( n + 2 ( 2 1 ) 2 ) ( 2 2 2 n + 3 2 1 ) a = \overline{AP_{\frac{n}{2}}} = (\sqrt{2} + (\dfrac{n}{2} - 1))x = (\dfrac{n + 2(\sqrt{2} - 1)}{2})(\dfrac{2\sqrt{2}}{2n + 3\sqrt{2} - 1})a =

2 ( n + 2 ( 2 1 ) ) 2 n + 3 2 1 a \dfrac{\sqrt{2}(n + 2(\sqrt{2} - 1))}{2n + 3\sqrt{2} - 1}a \implies A 2 = ( A P n 2 ) 2 = 2 ( n + 2 ( 2 1 ) ) 2 ( 2 n + 3 2 1 ) 2 a 2 A_{\triangle{2}} = (\overline{AP_{\frac{n}{2}}})^2 = \dfrac{2(n + 2(\sqrt{2} - 1))^2}{(2n + 3\sqrt{2} - 1)^2}a^2

A 1 + A 2 = 2 ( 2 n 2 + 6 ( 2 1 ) n + 5 ( 3 2 2 ) ) ( 2 n + 3 2 1 ) 2 a 2 A T = a 2 ( A 1 + A 2 ) \implies A_{\triangle{1}} + A_{\triangle{2}} = \dfrac{2(2n^2 + 6(\sqrt{2} - 1)n + 5(3 - 2\sqrt{2}))}{(2n + 3\sqrt{2} - 1)^2} a^2 \implies A_{T} = a^2 - (A_{\triangle{1}} + A_{\triangle{2}}) \implies

A T A A B C D = 8 n + 14 2 11 ( 2 n + 3 2 1 ) 2 = 32 2 41 9 \dfrac{A_{T}}{A_{\triangle{ABCD}}} = \dfrac{8n + 14\sqrt{2} - 11}{(2n + 3\sqrt{2} - 1)^2} = \dfrac{32\sqrt{2} - 41}{9}

( 128 2 164 ) n 2 + ( 860 620 2 ) n + 728 2 1064 = 0 \implies (128\sqrt{2} - 164)n^2 + (860 - 620\sqrt{2})n + 728\sqrt{2} - 1064 = 0 \implies

n = ( 860 620 2 ) ± ( 204 108 2 ) 2 ( 128 2 164 ) n = \dfrac{-(860 - 620\sqrt{2}) \pm (204 - 108\sqrt{2})}{2(128\sqrt{2} - 164)}

For ( + ) : n = 656 + 512 2 2 ( 128 2 164 ) = (+): n = \dfrac{-656 + 512\sqrt{2}}{2(128\sqrt{2} - 164)} = 4 ( 128 2 164 ) 2 ( 128 2 164 ) = 2 \dfrac{4(128\sqrt{2} - 164)}{2(128\sqrt{2} - 164)} = 2

For ( ) : n = 371 525 2 367 < 0 (-): n = \dfrac{371 - 525\sqrt{2}}{367} < 0 \therefore dropping negative irrational root n = 2 \implies \boxed{n = 2} .

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...