25.5.2 Sine and Tangent of a Sum or Difference
Core ideaFollowing the trail from the cosine formula, the sum-and-difference expansions for sine and tangent come along too.
Module goal. We build the rules for combining known angles into new ones — starting from the sum and difference formulas to reach double-angle and auxiliary-angle forms — so messy trig expressions can be simplified, combined, and evaluated.