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4. Understanding the formulas--Page 4
Using the Sum and Difference formulas, we can easily get the Derived formulas.
, if k is an odd integer,
, if k is an even integer.
Here T can be any trigonometric functions, and co-T is the co-function of function T. For example, sine and cosine are co-functions of each other, similarly tangent and cotangent, secant and cosecant respectively.
and so on, all of such formulas can proved by the Sum and Difference formulas:
since , and so on. In the same way we can prove all of them.
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