I think the answer would be c but I'm not positive
I hope I help :)
Answer:
a = ±10/9
Step-by-step explanation:

Answer: 20. 4, 3, 2, 1...
21. 17....
Step-by-step explanation:
Answer: 1, 3, and 4 can form triangles. The rest can not.
Step-by-step explanation:
on triangles 1, 3, and 4, the 2 smaller sides added together are greater than the longer side.