Geometry !
with 13 through 18

If c^2 > a^2 + b^2 then the triangle is obtuse.
If c^2 < a^2 + b^2 then the triangle is acute.
If c^2 = a^2 + b^2 then the triangle is a right triangle.
13. 7^2 __ 3^2 + 6^2
49 ___ 9 + 36
49 > 47 obtuse
14. 12^2 __ 4^2 + 11^2
144 ___ 16 + 121
144 > 137 obtuse
15. 26^2 ___ 24^2 + 10^2
676 ___ 576 + 100
676 = 676 right triangle
16. 15^2 ___ 12^2 + 9^2
225 ___ 144 + 81
225 = 225 right triangle
17. 31^2 __ 30^2 + 10^2
961 ___ 900 + 100
961 < 1000 acute triangle
18. 12^2 ___ 7^2 + 7^2
144 ___ 49 + 49
144 ___ 98
144 > 98 obtuse