The sequence of numbers named for this shape is generated by adding consecutive natural numbers.  
One unsolved problem in finding the number of these shapes is named for Kobon Fujimura. This   shape is commonly used to represent the first iteration of a Koch snowflake or a Sierpinski gasket.  The sequence of numbers named for this shape is generated by adding consecutive natural numbers.   The area of one of these shapes can be computed using Heron's formula, and its side lengths and  angle measures can be found using the Law of Sines or Law of Cosines. Two of these can be proved  to be similar by showing that two corresponding angles are congruent. Name these shapes whose  interior angles add up to 180 degrees and which have three vertices.
One unsolved problem in finding the number of these shapes is named for Kobon Fujimura.
triangles