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Abstract
Textbooks and reform documents provide a multiple source of descriptions of what it is to treat trigonometric functions as a school subject during three periods. Trigonometric functions in schools are introduced and developed in three major mathematical frames over the course of three reform periods in the United States unified mathematics, new math, and standards-based instruction. Those frames are triangle, circle, and vector. They are used to explain the variations in textbooks treatments of trigonometry as a phenomenon during and across reforms. Using phenomenology, I focused on ideas of trigonometric functions as a school subject manifested in textbooks and reform documents during reform periods. I used Schubrings methodology of historical textbook analysis to develop a history of interpretations of trigonometric functions as a school subject. I described the appearances and changes in the treatments of the trigonometric functions for each frame along the course of three reform periods from selected textbooks and reform documents.