Ricorda queste formule:
(senα)^2 + (cosα)^2 = 1;
cos2α = (cosα)^2 - (senα)^2;
sen2α = 2 senα cosα.
cos2α - [cosα sen2α] /senα =
= [(cosα)^2 - (senα)^2] - [cosα * 2 senα cosα] /senα =
= (cosα)^2 - (senα)^2 - (2 cosα cosα) =
= (cosα)^2 - (senα)^2 - 2 (cosα)^2 =
= - (cosα)^2 - (senα)^2 = - [(cosα)^2 + (senα)^2] = - 1.
Ciao @kristelpi