Should we avoid using trig functions other than sine, cosine, and tangent? Should we think of facts about them (e.g., the other Pythagorean identities) as non-curricular facts? How about their definitions?
The progressions doc says “Because the second three may be expressed as reciprocals of the first three, this progression discusses only the first three,” but it’s unclear whether that means these are being left out of the discussion to save space, or that they’re being left out of the expectations for students because they’re redundant.