The three angle of any triangle sum to 180 degrees so the angles of two triangles sum to 360 degrees - which is the angular measure around a point.
yes
If a tile is a square or rectangle, cut the tile from corner to corner and you will create two triangles.
Yes a triangle will tessellate leaving no gaps or overlaps
Yes.
Yes.
A Triangle with a perfectly 90 Degree angle, which also should have a corner which is perfectly square. :D
yes it does
No.
No
heptagon and an equilateral triangle
An [equilateral] triangle, square and hexagon are the only regular polygons which, by themselves, will tile a surface.
Yes.