no
There are 6 triangles in an octagon
Join the 6 triangles like pizza slices
8
No but they will make a hexagon
6
6 polygon triangles make a hexagon
1
You cannot make a solid shape with 5 rectangular sides. You can make an open rectangular box, leaving off the last side. You can make a rectangular box (like a cube, but with possibly unequal sides) with 6 rectangles. You can make a rectangular pyramid with one rectangle and four triangles.
4 triangles make 1 hexagon. (easy way to find out how many triangles there are in a polygon- take number of sides of the polygon, subtract 2 from it, that's your answer!)
6
Join them together
Sort of depends on how the two triangles are beforehand. One nice way to make four equilateral triangles from six matchsticks is to make a tetrahedron (a triangular pyramid).