coseno(a)=2/5

Usa el arcoseno:

$$\underset{\,\,\,\,^{\textcolor[rgb]{0.66,0.66,0.66}{360^\circ}}}{{cos}}^{\!\!\mathtt{-1}}{\left({\frac{{\mathtt{2}}}{{\mathtt{5}}}}\right)} = {\mathtt{66.421\: \!821\: \!521\: \!798^{\circ}}}$$

El ángulo ~66.422