  The simplest rigourous, non-circular proof that d(cos x) = -sen x. Some
details omitted if the gap is intuitive, nevertheles, each gap is easily
filled, rigourously. As Ehrenpreis and others have pointed out, the usual text
book `proof' is circular, since it assumes the area of circular sectors is
already known. Various Cours d'Analyse tomes (Hermite, Jordan, etc.) have
correct proofs which, however, are more difficult.
