Trigonometry is not necessarily the geometry of a circle, but every trigonometric function of an angle can be constructed/defined geometrically from the unit circle. It shows that trigonometry is linked to circles from the very start. There are other uses and definitions, however since all the functions can be defined on a unit circle, it shows that all the functions describe the geometry of the circle. So although Trigonometry is not used solely on circles, it still uses functions that have their basis in circles and so is technically always related to circular geometry. Since the relation is there it is a fair statement to say Trigonometry is about the Geometry of a Circle, but it is important to know that this is not the only application of this field.
Question: Can Trigonometry be said to be about anything else besides circles?
karolmath 