Will a regular isosceles triangle tessellate?

No, a regular isosceles triangle will not tessellate. In order for a shape to tessellate, it must be able to fit together with copies of itself without any gaps or overlaps. Regular isosceles triangles have angles of 90, 45, and 45 degrees, which do not allow for a repeating pattern that covers a plane without any spaces. Regular polygons that tessellate include equilateral triangles, squares, and hexagons.