bjr
(a + b + c)² = (a + b + c) x (a + b + c)
on développe
(a + b + c) x (a + b + c) =
a x (a + b + c) + b x (a + b + c) + c x (a + b + c) =
a² + ab + ac + ba + b² + bc + ca + cb + c² =
a² + b² + c² + ab + ba + ac + ca + bc + cb =
a² + b² + c² + 2ab + 2ac + 2bc
