Inspired by Ethan

Geometry Level 5

If the pattern of inscribed circles shown in the image above is repeated infinitely, each new circle is inscribed within a quadrant of the larger circle and the radius of the largest circle is 10, then what is the area of all the circles?

The area of all the circles can be expressed as a π ( b + c ) a\pi (\sqrt b + c) , where a , b a,b and c c are integers with b b square-free. Find a + b + c a+b+c .


The answer is 53.

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4 solutions

Hung Woei Neoh
Jun 25, 2016

Let us analyze the first two circles:

For easy naming, let us call the circles c 1 , c 2 , c 3 , c_1,c_2,c_3,\ldots . The radius of these circles will be r 1 , r 2 , r 3 , r_1,r_2,r_3,\ldots and the area of these circles will be a 1 , a 2 , a 3 , a_1,a_2,a_3,\ldots

First of all, since O B Q D OBQD is a quadrant, we know that A O C = 9 0 \angle AOC=90^{\circ} . Lines O C D OCD and O A B OAB are tangent to c 2 c_2 , so we also know that O C P = O A P = A P C = 9 0 \angle OCP = \angle OAP = \angle APC =90^{\circ} . We know that A P = C P = r 2 AP=CP=r_2 , therefore it is obvious that O A P C OAPC is a square. With this, we can find O P OP .

O P = C P 2 + A P 2 = r 2 2 + r 2 2 = 2 r 2 2 = 2 r 2 OP = \sqrt{CP^2+AP^2} = \sqrt{r_2^2+r_2^2} = \sqrt{2r_2^2} = \sqrt{2}r_2

Note that P Q = r 2 PQ = r_2 and O Q = r 1 = 10 OQ = r_1=10 . We can then say that

O Q = O P + P Q r 1 = 2 r 2 + r 2 r 2 = r 1 2 + 1 = 10 2 + 1 OQ = OP + PQ\\ r_1 = \sqrt{2}r_2+r_2\\ r_2 = \dfrac{r_1}{\sqrt{2}+1} = \dfrac{10}{\sqrt{2}+1}

Now, notice that for the circles c 2 c_2 and c 3 c_3 , c 3 c_3 and c 4 c_4 ...., the pattern repeats in the exact same manner.

This implies that r n + 1 = r n 2 + 1 r_{n+1} = \dfrac{r_n}{\sqrt{2}+1}

The radius of the circles, r 1 , r 2 , r 3 , r 4 , r_1,r_2,r_3,r_4,\ldots are 10 , 10 2 + 1 , 10 ( 2 + 1 ) 2 , 10 ( 2 + 1 ) 3 , 10,\dfrac{10}{\sqrt{2}+1},\dfrac{10}{(\sqrt{2}+1)^2},\dfrac{10}{(\sqrt{2}+1)^3},\ldots

The areas of the circles, a 1 , a 2 , a 3 , a 4 , a_1,a_2,a_3,a_4,\ldots are (use a = π r 2 a=\pi r^2 ): 100 π , 100 π ( 2 + 1 ) 2 , 100 π ( 2 + 1 ) 4 , 100 π ( 2 + 1 ) 6 , 100\pi,\dfrac{100\pi}{(\sqrt{2}+1)^2},\dfrac{100\pi}{(\sqrt{2}+1)^4},\dfrac{100\pi}{(\sqrt{2}+1)^6},\ldots

Notice that the areas of these circles form a geometric progression where a = 100 π a=100\pi and r = 1 ( 2 + 1 ) 2 r=\dfrac{1}{(\sqrt{2}+1)^2}

Now, we are looking for the sum of the areas of all the circles when this pattern is repeated infinitely. In other words, the sum of the GP above when n = n=\infty . Use the formula S = a 1 r S_{\infty} = \dfrac{a}{1-r} :

A = a 1 + a 2 + a 3 + a 4 + = 100 π 1 1 ( 2 + 1 ) 2 = 100 π 1 1 3 + 2 2 = 100 π 3 + 2 2 1 3 + 2 2 = 100 π ( 3 + 2 2 ) 2 + 2 2 = 100 π ( 3 + 2 2 ) ( 2 1 ) 2 ( 2 + 1 ) ( 2 1 ) = 50 π ( 3 2 + 4 3 2 2 ) 2 1 = 50 π ( 2 + 1 ) A = a_1+a_2+a_3+a_4+\ldots\\ =\dfrac{100\pi}{1-\frac{1}{(\sqrt{2}+1)^2}}\\ =\dfrac{100\pi}{1-\frac{1}{3+2\sqrt{2}}}\\ =\dfrac{100\pi}{\frac{3+2\sqrt{2}-1}{3+2\sqrt{2}}}\\ =\dfrac{100\pi(3+2\sqrt{2})}{2+2\sqrt{2}}\\ =\dfrac{100\pi(3+2\sqrt{2})(\sqrt{2}-1)}{2(\sqrt{2}+1)(\sqrt{2}-1)}\\ =\dfrac{50\pi(3\sqrt{2}+4-3-2\sqrt{2})}{2-1}\\ =50\pi(\sqrt{2}+1)

a = 50 , b = 2 , c = 1 , a + b + c = 50 + 2 + 1 = 53 \implies a=50,\;b=2,\;c=1,\;a+b+c=50+2+1=\boxed{53}

@Hung Woei Neoh

Perfect solution. This also explains how you got the relationship between the two radii. Great work!

Mehul Arora - 4 years, 11 months ago

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Thanks! ¨ \ddot\smile

Hung Woei Neoh - 4 years, 11 months ago

I rarely find solutions this detailed. Upvote!

Manuel Kahayon - 4 years, 11 months ago

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Thanks. I personally feel that the most important part is showing how to find the radius of the smaller circles, since this is a geometry question

Hung Woei Neoh - 4 years, 11 months ago

I'm glad I'm not the only one who struggles at drawing circles ;)

Dan Ley - 4 years, 8 months ago
Chew-Seong Cheong
Jun 25, 2016

Let the largest radius be r 0 = 10 r_0 = 10 , the next largest be r 1 r_1 , then r 2 r_2 and so on. Then we note that:

r 0 = r 1 + 2 r 1 r 1 = r 0 1 + 2 r 2 = r 0 ( 1 + 2 ) 2 r 3 = r 0 ( 1 + 2 ) 3 . . . . . . r k = r 0 ( 1 + 2 ) k \begin{aligned} r_0 & = r_1 + \sqrt 2 r_1 \\ \implies r_1 & = \frac {r_0}{1+\sqrt 2} \\ r_2 & = \frac {r_0}{(1+\sqrt 2)^2} \\ r_3 & = \frac {r_0}{(1+\sqrt 2)^3} \\ ... & \quad \ ... \\ \implies r_k & = \frac {r_0}{(1+\sqrt 2)^k} \end{aligned}

The area of all the circles is given by:

A = k = 0 π r k 2 = π r 0 2 k = 0 ( 1 1 + 2 ) 2 k = π r 0 2 k = 0 ( 1 3 + 2 2 ) k = π r 0 2 ( 1 1 1 3 + 2 2 ) = 100 π ( 3 + 2 2 2 + 2 2 ) = 50 π ( 3 + 2 2 1 + 2 ) = 50 π ( ( 3 + 2 2 ) ( 2 1 ) ( 1 + 2 ) ( 2 1 ) ) = 50 π ( 1 + 2 ) \begin{aligned} A & = \sum_{k=0}^\infty \pi r_k^2 \\ & = \pi r_0^2 \sum_{k=0}^\infty \left(\frac 1{1+\sqrt 2} \right)^{2k} \\ & = \pi r_0^2 \sum_{k=0}^\infty \left(\frac 1{3+2\sqrt 2} \right)^{k} \\ & = \pi r_0^2 \left(\frac 1{1-\frac 1{3+2\sqrt 2}} \right) \\ & = 100 \pi \left(\frac {3+2\sqrt 2}{2+2\sqrt 2} \right) \\ & = 50 \pi \left(\frac {3+2\sqrt 2}{1+\sqrt 2} \right) \\ & = 50 \pi \left(\frac {(3+2\sqrt 2)(\sqrt 2 -1)}{(1+\sqrt 2)(\sqrt 2 -1)} \right) \\ & = 50 \pi (1+\sqrt 2) \end{aligned}

a + b + c = 50 + 2 + 1 = 53 \implies a + b + c = 50 + 2 + 1 = \boxed{53}

The relation between radius of bigger circle ( x x ) and and radius of circle inscribed ( r r ) is: r = x 1 + 2 . r=\dfrac{x}{1+\sqrt{2}}.

Radius of first circle= 10 10 [ Given \text{Given} ]

Radius of next circle= 10 1 + 2 \dfrac{10}{1+\sqrt{2}}

Radius of next circle= 10 1 + 2 1 + 2 = 10 ( 1 + 2 ) 2 \dfrac{\frac{10}{1+\sqrt{2}}}{1+\sqrt{2}}=\dfrac{10}{(1+\sqrt{2})^2}

\cdot \cdot \cdot \cdot \cdot \cdot

As we know area of circle is: π r 2 \pi \color{#20A900}{r^2}

Area of all circles: π ( 10 ) 2 + π ( 10 1 + 2 ) 2 + π ( 10 ( 1 + 2 ) 2 ) 2 + π ( 10 ( 1 + 2 ) 4 ) 2 + \Rightarrow \pi\color{#20A900}{(10)^2}+\pi \color{#20A900}{\left(\dfrac{10}{1+\sqrt{2}}\right)^2}+\pi \color{#20A900}{\left(\dfrac{10}{(1+\sqrt{2})^2}\right)^2}+\pi \color{#20A900}{\left(\dfrac{10}{(1+\sqrt{2})^4}\right)^2}+\cdot \cdot \cdot \cdot

π [ 100 + 100 ( 1 ( 1 + 2 ) 2 + 1 ( 1 + 2 ) 4 + 1 ( 1 + 2 ) 6 + ) ] \implies \pi\left[100+100\left(\color{#EC7300}{\dfrac{1}{(1+\sqrt{2})^2}+\dfrac{1}{(1+\sqrt{2})^4}+\dfrac{1}{(1+\sqrt{2})^6}+\cdot \cdot \cdot \cdot } \right)\right]

Using Geometric progression.

π [ 100 + 100 ( 1 ( 1 + 2 ) 2 1 ) ] \implies \pi\left[100+100\left(\color{#EC7300}{\dfrac{1}{(1+\sqrt{2})^2-1}}\right)\right]

π [ 100 + 100 ( 1 2 2 + 2 ) ] \implies \pi\left[100+100\left(\dfrac{1}{2\sqrt{2}+2}\right)\right]

π [ 100 + 50 ( 2 1 ) ] \implies \pi [100+50(\sqrt{2}-1)]

50 π ( 1 + 2 ) \implies 50\pi(1+\sqrt{2})

a + b + c = 50 + 2 + 1 = 53 \therefore a+b+c=50+2+1=\boxed{53}

@Abhay Kumar You only showed how to calculate the final answer. I urge you to show your working.

Mehul Arora - 4 years, 11 months ago

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Done.Is it ok? :-)

A Former Brilliant Member - 4 years, 11 months ago

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I wanted you to show how you ended up getting the relationship of the radii of the two circles, which makes the solution complete. You could refer Hung Woei Neoh's solution.

Mehul Arora - 4 years, 11 months ago
Nibedan Mukherjee
Jun 26, 2016

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