When to use what

With so many families of formulae available, the difficulty is choosing the right one. Here is a quick guide.

In summary — When to use what

• Addition/double-angle/half-angle formulae: when you need to move an argument from $(\alpha\pm\beta)$ to the individual summands, or to reduce $2\alpha$ and $\alpha/2$ to $\alpha$.
• Parametric formulae: when the equation is linear in $\sin$ and $\cos$ and you want to reduce it to a rational one in a single unknown $t=\tan(\alpha/2)$.
• Werner formulae: when you have a product of sines/cosines to integrate or simplify.
• Sum-to-product formulae: when you have a sum/difference of sines or cosines to factorise (typical in equations).

Links

Topics: Prostaferesi werner
Concepts: Formule di addizione · Formule di prostaferesi · Formule di werner · Formule parametriche
Functions: Coseno · Seno
Skills: Usare formule