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== Condition for intersection ==
If ''AD'', ''BD'', ''CD'' are assigned according to the arrangement,
:<math>AD \le BD \le CD</math>
Then ''AD, BD, CD'' intersect if and only if,
:<math>AD+BD \ge AB \ge BD-AD</math>
:<math>AD+CD \ge AC \ge CD-AD</math>
:<math>BD+CD \ge BC \ge CD-BD</math>
Viz, if
''AD''=r<sub>A</sub>=radius of sphere centered at '''A''',<br />▼
: ''
: ''
such that, <br />
:<math>r_A \le r_B \le r_C</math>
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