The complete list of product-to-sum trigonometric formulas are as follows:
The product-to-sum formulas can be obtained by observing that the sum and difference formulas for sine and cosine look very similar except for opposite signs in the middle. Then by combining the expressions, we can cancel terms.
The right-hand side is a product of cosine terms, while the left-hand side is a sum of cosine terms.
Using and identities, we have
We know that Substituting and , we get
Express as a sum of basic trigonometric functions (the solution should not include any products of trigonometric functions).
The product-to-sum formulas can be helpful in solving integration problems involving the product of trigonometric ratios.
This problem may seem tough at first, but after using the product-to-sum trigonometric formula, this integral very quickly changes into a standard form.
Using the first formula stated above, the integral is equivalent to
where is the constant of integration.