Answer:
6 cm to 10.9 cm or 14.8 cm to 20 cm
Explanation:
The longest of the two given sides can be the longest side, or it can be the second-longest side.
If 13 cm is the second-longest side, the triangle will be a right triangle for the longest side being ...
√(13² +7²) = √218 ≈ 14.76
That is, if the longest side is 14.8 cm up to the maximum of 13+7 = 20 cm, the triangle will be obtuse.
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Similarly, if 13 cm is the longest side, then the other side in a right triangle will be ...
√(13² -7²) = √120 ≈ 10.954
For a side length shorter than this down to 13-7 = 6 cm, the triangle will also be obtuse.
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The triangle will be obtuse for a long side between 14.8 and 20 cm, or for a third side between 6 and 10.9 cm.