2. Sum-difference product formula: sine+sine, sine first; Sine-sine, sine in the back; Cosine+cosine, cosine side by side; Cosine-cosine, cosine aside. Product and difference and difference product are opposites, so there is no need to remember formulas, and too many formulas are easy to be confused.
This formula is very important and commonly used in mathematics (common sense). It's easy to remember. You don't have to memorize it. Just remember the extensions of cos(A+B), COS(A-B) and sin. When you use it, just think about these expansions in your mind, simply add and subtract, and then multiply and divide by 1/2. It takes 1 2 seconds to figure it out.
Extended data
Sum-difference product formula: including sine, cosine, tangent, cotangent and difference product formula, which is a set of identities in trigonometric function, and sum-difference product formula *** 10 group. When sum-difference product is applied, it must be a trigonometric function with the same name (except tangent cotangent). If it is a synonym, it must be formulated into the same name by induction; If it is a high-order function, it must be reduced once by the power reduction formula.
The final result of product sum and difference is sum or difference; If two terms are multiplied and the latter is a cos term, the result of multiplication and difference is the addition of two terms; If not, the result is the subtraction of two terms; If two terms are multiplied, one is sin and the other is cos, then the results of multiplication and difference are both sin terms; If two terms are multiplied and both terms are sin, the result of product sum and difference will be preceded by a negative sign.