3,4,5 - Yes. 3+4>5 and 4+5>3 and 3+5>4. Also, this is a special right triangle (Pythagorean Triple).
4,5,9 - No. 4+5=9.
5,6,12 - No. 5+6<12
3.5, 4.5, 7 - Yes. 3.5+4.5>7 and 7+4.5>3 and 7+3.5>4.5
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