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Colles de mathématiques

Somme trigo & binomiale (ter)

Calculer $\cos a+\lp\begin{array}{c}n\\1\enar\rp\cos(a+b)+\lp\begin{array}{c}n\\2\enar\rp\cos(a+2b)+\dots +\lp\begin{array}{c}n\\n\enar\rp\cos(a+nb)$