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The product-to-sum identities [28] or prosthaphaeresis formulae can be proven by expanding their right-hand sides using the angle addition theorems. Historically, the first four of these were known as Werner's formulas, after Johannes Werner who used them for astronomical calculations. [29]
The six trigonometric functions are defined for every real number, except, for some of them, for angles that differ from 0 by a multiple of the right angle (90°). Referring to the diagram at the right, the six trigonometric functions of θ are, for angles smaller than the right angle:
Sum and difference: Find the sum and difference of the two angles. Average the cosines : Find the cosines of the sum and difference angles using a cosine table and average them, giving (according to the second formula above) the product cos α cos β {\displaystyle \cos \alpha \cos \beta } .
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Another twelve identities follow by cyclic permutation. The proof (Todhunter, [1] Art.49) of the first formula starts from the identity = , using the cosine rule to express A in terms of the sides and replacing the sum of two cosines by a product. (See sum-to-product identities.)
A double-dot product for matrices is the Frobenius inner product, which is analogous to the dot product on vectors. It is defined as the sum of the products of the corresponding components of two matrices and of the same size: : = ¯ = = ().
Another method of deriving vector and tensor derivative identities is to replace all occurrences of a vector in an algebraic identity by the del operator, provided that no variable occurs both inside and outside the scope of an operator or both inside the scope of one operator in a term and outside the scope of another operator in the same term ...
In mathematics, summation is the addition of a sequence of numbers, called addends or summands; the result is their sum or total.Beside numbers, other types of values can be summed as well: functions, vectors, matrices, polynomials and, in general, elements of any type of mathematical objects on which an operation denoted "+" is defined.