A square-based pyramid
A pentagon for a base, five triangles reaching up to a point on top.
It can, but it doesn't have to. The base can be any polygon, as long as the sides are triangles that come to a point at the top.
A pyramid - it has a square base, and four triangles meeting to a point at the top.
Pyramid with a square base. One square side on the bottom and four triangles meeting at a point on the top.
Postulates is, I believe, the starting point of reasoning that two triangles are the same.
a pentagonal pyramid it has five triangular sides that all meet at a point at the top. the base is a pentagon
Pyramid
Four of them
A pyramid
orthocenter
Draw lines from point to point and now you have six triangles to calculate