F'(x) = (a cos x - b sin x) .ex + (a sin x + b cos x ) .ex = ex.[cos x ( a + b ) + sin x( a - b )] F'(x) = f(x)
↔ ex.[cos x ( a + b ) + sin x( a - b )] = ex .cos x
↔ cos x ( a + b ) + sin x( a - b ) = 1.cos x + 0.sin x
↔ a + b = 1 v a - b = 0
↔ a = b = 1/2
2