It's a lot of circles!

Level pending

Extend the above diagram to n n congruent circles.

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 .

Find the integer value of n n for which A A J P 1 I A A B C D = 24 16 2 9 \dfrac{A_{AJP_{1}I}}{A_{ABCD}} = \dfrac{24 - 16\sqrt{2}}{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 13, 2020

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

Extending the diagram to 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

A A J P 1 I = x 2 = 8 4 n 2 + 4 ( 3 2 1 ) n + 19 6 2 a 2 \implies A_{AJP_{1}I} = x^2 = \dfrac{8}{4n^2 + 4(3\sqrt{2} - 1)n + 19 - 6\sqrt{2}}a^2 \implies

A A J P 1 I A A B C D = 8 4 n 2 + 4 ( 3 2 1 ) n + 19 6 2 = 24 16 2 9 = 8 ( 3 2 2 ) 9 \dfrac{A_{AJP_{1}I}}{A_{ABCD}} = \dfrac{8}{4n^2 + 4(3\sqrt{2} - 1)n + 19 - 6\sqrt{2}} = \dfrac{24 - 16\sqrt{2}}{9} = \dfrac{8(3 - 2\sqrt{2})}{9}

After simplifying we obtain:

4 ( 3 2 2 ) n 2 + 4 ( 11 2 15 ) n + 72 56 2 = 0 4(3 - 2\sqrt{2})n^2 + 4(11\sqrt{2} - 15)n + 72 - 56\sqrt{2} = 0 \implies

n = 11 2 + 15 ± 3 3 2 2 2 ( 3 2 2 ) n = \dfrac{-11\sqrt{2} + 15 \pm 3\sqrt{3 - 2\sqrt{2}}}{2(3 - 2\sqrt{2})}

( 2 1 ) 2 = 3 2 2 n = 11 2 + 15 ± 3 ( 2 1 ) 2 ( 3 2 2 ) (\sqrt{2} - 1)^2 = 3 - 2\sqrt{2} \implies n = \dfrac{-11\sqrt{2} + 15 \pm 3(\sqrt{2} - 1)}{2(3 - 2\sqrt{2)}}

For + + we obtain n = 4 ( 3 2 2 ) 2 ( 3 2 2 ) = 2 n = \dfrac{4(3 - 2\sqrt{2})}{2(3 - 2\sqrt{2})} = 2

For - we obtain: n = 9 7 2 3 2 2 = ( 9 7 2 ) ( 3 + 2 2 ) = 1 3 2 < 0 n = \dfrac{9 - 7\sqrt{2}}{3 - 2\sqrt{2}} = (9 - 7\sqrt{2})(3 + 2\sqrt{2}) = -1 - 3\sqrt{2} < 0 \therefore dropping the negative irrational root we obtain n = 2 n = \boxed{2} .

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...