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From: Thomas 'PointedEars' Lahn <PointedEars@web.de>
Newsgroups: comp.lang.javascript
Subject: Re: Refreshing grade school math.
Supersedes: <1650540.VLH7GnMWUR@PointedEars.de>
Date: Tue, 14 Jan 2020 01:01:37 +0100
Organization: PointedEars Software (PES)
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Xref: csiph.com comp.lang.javascript:38071

Michael Haufe (TNO) wrote:

> In elementary/primary school you should have learned the equation for a
> circle:
> 
> x^2 + y^2 = r^2
> 
> Alternate form to solve for y:
> 
> y = +/- sqrt(r^2 - x^2)
> 
> That's enough for you to plot your points.
> 
> Assuming a circle of radius 5 and centered at the origin (0,0)
> 
> iterate x from -r to +r and solve for y:
> 
> y = +/- sqrt((5)^2 - (-5)^2)
>   = +/- sqrt(25 - 25)
>   = +/- sqrt(0)
>   = +/- 0
>   = 0
> y = +/- sqrt((5)^2 - (-4)^2)
>   = +/- sqrt(25 - 16)
>   = +/- sqrt(9)
>   = +/- 3
> y = +/- sqrt((5)^2 - (-3)^2)
>   = +/- sqrt(25 - 9)
>   = +/- sqrt(16)
>   = +/- 4
> ...
> 
> now convert that to JavaScript

let r = 5;
document.body.appendChild(
  (new DOMParser()).parseFromString(
    `<svg xmlns="http://www.w3.org/2000/svg" width="${2 * r + 2}" 
height="${2 * r + 2}" stroke="black"><circle cx="${r + 1}" cy="${r + 1}" 
r="${r}" 
fill="transparent"/></svg>`,
    'image/svg+xml'
  ).documentElement
);

;-b

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