55 lines
1.8 KiB
Markdown
55 lines
1.8 KiB
Markdown
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---
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id: 5900f5191000cf542c51002b
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title: 'Problem 428: Necklace of Circles'
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challengeType: 5
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forumTopicId: 302098
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dashedName: problem-428-necklace-of-circles
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---
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# --description--
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Let `a`, `b` and `c` be positive numbers.
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Let W, X, Y, Z be four collinear points where |WX| = `a`, |XY| = `b`, |YZ| = `c` and |WZ| = `a` + `b` + `c`.
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Let C<sub>in</sub> be the circle having the diameter XY.
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Let C<sub>out</sub> be the circle having the diameter WZ.
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The triplet (`a`, `b`, `c`) is called a *necklace triplet* if you can place `k` ≥ 3 distinct circles C<sub>1</sub>, C<sub>2</sub>, ..., C<sub><var>k</var></sub> such that:
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<ul><li>C<sub><var>i</var></sub> has no common interior points with any C<sub><var>j</var></sub> for 1 ≤ <var>i</var>, <var>j</var> ≤ <var>k</var> and <var>i</var> ≠ <var>j</var>,</li><li>C<sub><var>i</var></sub> is tangent to both C<sub>in</sub> and C<sub>out</sub> for 1 ≤ <var>i</var> ≤ <var>k</var>,</li><li>C<sub><var>i</var></sub> is tangent to C<sub><var>i</var>+1</sub> for 1 ≤ <var>i</var> < <var>k</var>, and</li><li>C<sub><var>k</var></sub> is tangent to C<sub>1</sub>.</li></ul>
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For example, (5, 5, 5) and (4, 3, 21) are necklace triplets, while it can be shown that (2, 2, 5) is not.
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<img src="https://projecteuler.net/project/images/p428_necklace.png" alt="a visual representation of a necklace triplet" />
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Let T(`n`) be the number of necklace triplets (`a`, `b`, `c`) such that `a`, `b` and `c` are positive integers, and `b` ≤ `n`. For example, T(1) = 9, T(20) = 732 and T(3000) = 438106.
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Find T(1 000 000 000).
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# --hints--
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`necklace(1000000000)` should return 747215561862.
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```js
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assert.strictEqual(necklace(1000000000), 747215561862);
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```
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# --seed--
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## --seed-contents--
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```js
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function necklace(n) {
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return true;
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}
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necklace(1000000000)
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```
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# --solutions--
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```js
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// solution required
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```
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