Answer:
A. 1,2,3
Explanation:
The only Pythagorean triple that is an arithmetic sequence is 3, 4, 5 and its multiples.
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The triple 1, 2, 3 is not a Pythagorean triple. (It doesn't even form a triangle.)
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If you don't recognize the offered choices as being among the Pythagorean triples you have memorized, you can always check to see if ...
a² +b² = c²
1² +2² = 1 +4 = 5 ≠ 3² . . . . . (1, 2, 3) is not a Pythagorean triple
3² +4² = 9 +16 = 25 = 5²
5² +12² = 25 +144 = 169 = 13²
7² +24² = 49 +576 = 625 = 25²
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A couple of other triples commonly seen are ...
(8, 15, 17), (9, 40, 41)