Unit Circle Worksheet and Answer Key - mathwarehouse Free worksheet(pdf) and answer key on Unit Circle. 25 scaffolded questions that start relatively easy and end with some real challenges. Plus model problems explained step by step Unit Circle w/ Everything (Charts, Worksheets, 35+ Examples) Well, the Unit Circle, according to RegentsPrep, is a circle with a radius of one unit, centered at the origin. Why make a circle where the radius is 1, you may ask? If the radius is a length of 1, then that means that every Reference Triangle that we create has a hypotenuse of 1, which makes it so much easier to compare one angle to another.