Answer: yes, yes, no, yes
Explanation:
triangles are unique in that they're the only shape whose angles add up to 180º. So for this problem, add up all the angles, and if they equal 180º you've got a triangle.
84 + 47 + 49 = 180, so triangle
21 + 57 + 102 = 180, so triangle
55 + 25 + 65 = 145, so no triangle
80 + 70 + 30 = 180, so triangle