# 8191: Trigonometry - Double angle formulae - Visualise cos(ax) Formulae

The double angle formulae are useful in solving trigonometric formulae. This display allows you to view formulae relating to $\color{green}{\cos (a \, x)}$ with $\color{green}{a}$ an adjustable constant.

By adjusting $\color{green}{a}$, $\color{red}{b}$, $\color{red}{c}$ and $\color{red}{d}$ to make the green and red curves coincide, you can view the double angle formulae, for $\color{green}{\cos (2 \, x)}$ in its different forms.