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moocher

um....... right. maybe!

edit, I dont really understand any of your message :)

I think I'd like the midline to touch the circle. Would an example help? My planks are 30cm wide and I want to make a circle of 240cm diameter, so 9 planks in total (7 full planks wide, with two half planks at each edge). P1 = ~~270cm~~ 240cm, P2 =?

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useless in reply to @moocher
No trouble - the formula just tells you the relationship between the x & y coordinates of a point on the circumference. We can just look at the top-right bit as Clockwise said, and stick to positive x & y (it's simpler if we just say the origin 0,0 is in the centre of the circle).

So, x is the sideways distance from the centre of the circle, to the middle of the plank we want to measure.
That leaves y as the height of the top of the plank above the middle of the circle. We actually want 2y in the end, because there's the same length below the centre as above, but solving the equation for y is how we get there.

OK, so D=240, and r=120 (since this formula uses radius instead of diameter).
I assume that means the centre plank should be 240 long?

If so, the next plank to the right has x=30 (the centreline is 30cm right of the middle of the circle), and
```
           x^2 + y^2 = r^2
becomes   30^2 + y^2 = 120^2
                   y = √(120^2 - 30^2)
```
and then we repeat the whole thing for x=60 for the second plank, etc.
```
plank#   x        y        height
0        0        120      240
1        30       116      232
2        60       104      208
3        90       79       159
```
So you'd have one plank #0 in the centre, 2 of plank #1 (just right and left of the centre), etc.

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