Was looking up simple geometry for the equilateral triangle and came across it enclosed in circles. This has got to be a personal favourite of mine, so much balance in one simple design.
I can actually see how I could build a lot of shapes from this one design. The way the angles intersect and the negative space in this geometry could be used as a positive shape to create something totally new.
This design also encapsulates one of the principals of sacred geometry that I really love as a designer. That is that you must balance you straight edges to curved and also positive and negative shapes