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I want to draw, say, 8 smaller circles that are adjacent to the big circle the edge of a big circle, similar to this picture. enter image description here

I know the center coordinates of the bigger circle $(A, B)$, its radius $(R)$,radius of the smaller circles $(r)$, and the number of circles I want to draw $(n)$.

My question is very similar to the one discussed there, with one exception. I want a formula that calculates center coordinates of circles adjacent, not those on the edge of a bigger circle. Mathematics is not my strongest side (to say the least), so I'd greatly appreciate any help. Thank you!

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  • $\begingroup$ I think $n$ determines $r$, so for example if you want $6$ externally tangent circles then $r$ is fixed at $r=R$ (all $7$ circles are the same size in that special case). In general, you could write $r$ as a function of $n$ and it would be strictly decreasing. $\endgroup$ Commented Jan 14, 2019 at 20:13
  • $\begingroup$ I mention that because you cannot presuppose $n$ and $r$ - most pairs that you choose will lead to impossible situations. Just presuppose $n$ and $R$ and those will together determine $r$. $\endgroup$ Commented Jan 14, 2019 at 20:14
  • $\begingroup$ Thank you, that makes sense (in lay terminology, only certain amount of circles of size will fit, correct?) $\endgroup$ Commented Jan 14, 2019 at 20:16
  • $\begingroup$ Connect the centers of the $n$ externally tangent circles and you will have a regular $n$-gon $\endgroup$ Commented Jan 14, 2019 at 20:16
  • $\begingroup$ If you choose the number of small circles and the size of the big circle, the size of the small circles is fixed, invariable, set, locked. $\endgroup$ Commented Jan 14, 2019 at 20:17

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For the 'adjacent' circles use : $\delta = 360/n$ where $n$ is the number of circles you want. Then the centers are $c_i = ((R+r)\cos i\delta + \phi, (R+r)\sin i\delta + \phi)$, where $i=0,1,...,n-1$ and $\phi$ is some offset rotation. Note that the small circles will not necessarily touch, but they will touch the large circle.

Edit: Here's a shadertoy example: https://www.shadertoy.com/view/wsfGWj

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  • $\begingroup$ I just edited it, realized I had already divided by $n$, so check the corrected version. @AntonLeontyev $\endgroup$
    – lightxbulb
    Commented Jan 14, 2019 at 20:25
  • $\begingroup$ Could you please clarify what do you mean by "offset rotation"? Thank you! $\endgroup$ Commented Jan 14, 2019 at 20:29
  • $\begingroup$ @AntonLeontyev check the shadertoy example I gave, the offset is literally what rotates the circles around. $\endgroup$
    – lightxbulb
    Commented Jan 14, 2019 at 20:41

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