Not Nine Circles Theorem

Geometry Level pending

In the square, five identical pink circles and four quarter circles are positioned as shown. If the radius ratio of one quarter circle to one pink circle can be expressed as

A + B C + D E + F G A + B\sqrt{C} + D\sqrt{E + F\sqrt{G}}

where A A , B B , C C , D D , E E , F F , and G G are positive integers and C C and G G are square free. Input A + B + C + D + E + F + G A + B + C + D + E + F + G as your answer.


The answer is 13.

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2 solutions

Sathvik Acharya
Feb 1, 2021

Let the radius of the four pink {\color{#E81990}\text{pink}} circles be 1 1 and the radius of the four quarter circles be r r . So, the radius ratio of one quarter circle to one pink circle is r 1 = r \dfrac{r}{1}=r .

In D E F \triangle DEF , D E 2 = E F 2 + F D 2 ( 1 + r ) 2 = 1 2 + F D 2 F D = r 2 + 2 r \begin{aligned} DE^2&=EF^2+FD^2 \\ (1+r)^2&=1^2+FD^2 \\ \implies FD&=\sqrt{r^2+2r} \end{aligned} The length of each side of the square is, A D = A F + F D = 1 + r 2 + 2 r AD=AF+FD=1+\sqrt{r^2+2r} In D C B \triangle DCB , D B = r + r 2 , B C = 1 + r 2 + 2 r , C D = 1 + r 2 + 2 r DB=r+r-2,\;\; BC=1+\sqrt{r^2+2r},\;\; CD=1+\sqrt{r^2+2r} D B 2 = B C 2 + C D 2 ( 2 r 2 ) 2 = ( 1 + r 2 + 2 r ) 2 + ( 1 + r 2 + 2 r ) 2 2 r 2 = 2 ( 1 + r 2 + 2 r ) \begin{aligned} DB^2&=BC^2+CD^2\\ (2r-2)^2&=(1+\sqrt{r^2+2r})^2+(1+\sqrt{r^2+2r})^2 \\ \implies 2r-2&=\sqrt{2}\left(1+\sqrt{r^2+2r}\right) \end{aligned} Solving the above equation, we have r = 3 + 2 + 2 2 + 2 r=3+\sqrt{2}+2\sqrt{2+\sqrt{2}} Therefore, A = 3 , B = 1 , C = 2 , D = 2 , E = 2 , F = 1 , G = 2 A + B + C + D + E + F + G = 13 A=3,\;B=1,\;C=2,\;D=2,\;E=2,\;F=1,\;G=2\implies A+B+C+D+E+F+G=\boxed{13}

K T
Feb 1, 2021

Suppose the square has side 1, the large circles have radius R, the small circles have radius r. Half a diagonal is 1 2 2 = R r \frac{1}{2}\sqrt{2}=R-r We can also identify a right-angled tiangle with hypothenuse R+r and right sides 1-r and r so that ( R + r ) 2 = ( 1 r ) 2 + r 2 (R+r)^2=(1-r)^2+r^2 Combining these we get 2 r 2 + ( 2 2 + 2 ) r 1 4 = 0 2r^2+(2\sqrt{2}+2)r-\frac{1}{4}=0 Taking the positive root: r = 1 2 8 + 4 1 2 2 1 2 , R = 1 2 8 + 4 1 2 r=\frac{1}{2}\sqrt{\sqrt{8}+4}-\frac{1}{2}\sqrt{2}-\frac{1}{2},R=\frac{1}{2}\sqrt{\sqrt{8}+4}-\frac{1}{2}

The ratio can, after some work, be reduced to R r = 3 + 2 + 2 2 + 2 \frac{R}{r}=3+\sqrt{2}+2\sqrt{2+\sqrt{2}} Here we have A = 3 , B = 1 , C = 2 , D = 2 , E = 2 , F = 1 , G = 2 A=3,B=1,C=2,D=2,E=2,F=1,G=2 These numbers have sum of 13 \boxed{13}

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