Triangle their internal angles, measured here in degrees. A triangle that has two angles with the same measure also has two sides with the same length, and therefore it is an isosceles triangle. It follows that in a triangle where all angles have the same measure, all three sides have the same length, and such a triangle is therefore equilateral. Triangles are assumed to be two-dimensional plane figures, unless the context provides otherwise (see Non-planar triangles, below). In rigorous treatments, a triangle is therefore called a "2-simplex" (see also Polytope). Elementary facts about triangles were presented by Euclid in books 1–4
Venus Equilateral by a low-power source. Powered by batteries, the driver tube enables the multi-G constant-boost spaceships seen throughout the series. At the series opening both Mars and Venus have been colonized. Both are described as having shirt-sleeve environments in at least some regions. Eleven of the thirteen Venus Equilateral stories were published in "Astounding Science Fiction" between 1942 and 1945. They were: Smith wrote a 12th story, "Mad Holiday", for the first book collection "Venus Equilateral" (1947) (which omitted "Lost Art" and "Identity"), and a 13th story, "The External Triangle", for Harry Harrison's 1973 anthology "Astounding". All 13 stories were included