Passing through each other's centers

Geometry Level 1

The figure at right consists of 3 identical circles which pass through each others' centers.

If the thicker, outer perimeter has length 12, what is the circumference of one of the 3 identical circles?

6 7 8 9

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16 solutions

In the figure, since the circles are identical, they have same radius.

So Δ P F B , Δ P B D , Δ P G D \Delta PFB, \Delta PBD,\Delta PGD are equilateral.

Therefore angle G P F = 180 GPF=180

So Arc FG is a semicircle of perimeter 1 3 × 12 = 4 \frac{1}{3}\times12=4

Therefore the circumference of any one of the 3 identical circles is 8 \boxed 8 .

excellently solved.

Mohammad Khaza - 3 years, 3 months ago

Perfect, Thank you!

beyond measures - 3 years, 3 months ago
Suresh Jh
Feb 8, 2018

Thanks for the visual

beyond measures - 3 years, 3 months ago

Because all circles pass through each other's centers, the total perimeter consists of 3 half circles, so a half circle is 12/3 = 4. The whole perimeter of 1 circle must be 8.

Moderator note:

Note this proof is incomplete: it doesn't justify why the circle portions are half-circles.

I love the simplicity of this explanation.

Scott Rudy - 3 years, 3 months ago

That is simple

Samuel Gurevich - 3 years, 3 months ago

@challenge master: it was a quick drop of a solution. Proofed the why for myself with equilateral triangles like @Shreyansh Mukhopadhyay did above.

Peter van der Linden - 3 years, 3 months ago

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Yes - and this kind of solution can be valuable - just want to be clear for readers what's going on.

Jason Dyer Staff - 3 years, 3 months ago

Because when the circles are touching the centres of each one, the portion become the half. This is a generic rule.

Adeeb Khuda - 3 years, 3 months ago

I disagree. The reasoning is as conclusive as that of the original problem. Saying that a circle is two semicircles would had been unnecessary. I think

F C - 3 years, 3 months ago

Challenge master was just noting valuable info in reguards to other valuable info

beyond measures - 3 years, 3 months ago
Mohammad Khaza
Feb 12, 2018

as,all circles pass through each others centers,the total perimeter is actually for \text{as,all circles pass through each others centers,the total perimeter is actually for } 1 2 + 1 2 + 1 2 = 3 2 \frac{1}{2}+\frac{1}{2}+\frac{1}{2}=\frac{3}{2} circles \text{circles}

we know that ,the perimeter of a circle is \text{we know that ,the perimeter of a circle is} = 2 τ r =2τr

now, \text{now,}

2 τ r × 3 2 = 12 2τr \times \frac{3}{2}=12

or, 2 τ r = 12 × 2 3 2τr=12 \times \frac{2}{3} = 8 8 square unit. .............[answer]

Robert DeLisle
Feb 17, 2018

Scott Blankenship
Feb 17, 2018

As the circles pass through each others centres, the intersection of any two circles that is not on the bold line must be the centre of a third circle.

As a result, the bold line is 3 semi-circles, having a perimeter of 12. Therefore the circumference of each circle is (12/3)*3 = 8.

Josiah Gillispie
Feb 13, 2018

https://sciencevsmagic.net/geo/#0A1.1A0.2A0.3L4

The line here is drawn from the intersection points of the other two circles. That it goes through the center, and is therefore a diameter, gives us a crucial step: each circle is contributing half of its circumference to the circumference of the whole shape. There are three circles. The rest is left as an exercise for the reader.

Nice website! You should promote your website on this site too .

Pi Han Goh - 3 years, 3 months ago
Fergus Babb
Jun 30, 2018

The figure can be drawn with three regular hexagons inscribed within each circle part, the middle most equilateral triangle (1/6 of the regular hexagon) shared between all the triangles, and the three that share an edge with the central most triangle are shared between two of the hexagons.

Remaining, each overlapped circle has their separate 1/2 hexagon (with outer perimeter (ignoring the flat surface that it has been halved across) also half that of the full hexagon). Because these outer half hexagons are inscribed in the circle, and are regular, we notice that the part of the circle (which is included in the perimeter which = 12) starts and ends at what we now know are opposing points of the hexagon, (or vertices of the equilateral triangle we formed by adding three congruent triangles to the centre most one) so is a semi circle.

We can now do:

12/3 (Full perimeter/ Parts of that perimeter) = 4, which is the curved edge of the semi circle (half of the full circle), so 4*2 which =8

Hence 8

Jay Dalal (Cms)
Feb 15, 2018

The three circles are identical, so you can split the total perimeter of the thick line into three. 1. 12/3 = 4 Then, I assumed that half of one circle’s circumstance is thick and half is thin, so I multiplied my result by 2 to receive my answer. 2. 4x2= 8 8 is the circumstance of one identical circle

Yash Ghaghada
Feb 16, 2018

Just imagine my sayings

From the given diagram just remove one circle form bottom two, and in mirror see them now you can see that the circles on the thicker side are half circles

Reyna McCartney
Feb 14, 2018

My clumsy solution is this: The thicker perimeter consists of 3 halves of the circumference of the circles (c), so 3/2c=12. Therefore the circumference of 1 circle is 12÷3/2=8

Nothing clumsy about that!

For completeness, can you prove that they are 3 halves of a circumference?

Pi Han Goh - 3 years, 3 months ago

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Since the circles are identical and pass through each others centers the outer perimeter must consist of halves of the circumference of the circles. It kind of makes intuitive sense to me, but I haven't learnt the geometry in depth yet, so can't derive a formula for it.

Reyna McCartney - 3 years, 3 months ago
Gaurang Mohta
Feb 13, 2018

As the shape is symmetrical, the perimeter can be divided into 3 equal semicircles each having perimeter = 4. So circumference of any circle is 2*4=8

Parveen Kohli
Feb 13, 2018

because all the circles are passing through each other's centers so that they cut the circumference of each other into half that's why we have the total circumference 1/2+1/2+1/2=3/2 3/2 of circumference is 12 so that 2/3*12=8 is the right answer

Mike Young
Feb 13, 2018

3 1/2 circles totaling 12 divided by 3 equals 4 x 2 equals 8

Dan Moulding
Feb 12, 2018

2(12/3) or 12*2/3

Gaytri Kapoor
Feb 12, 2018

As the perimeter is sum of three semicircles so r comes out to be14/11 then using 2pi r we getthe answer 8

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