f 0 1 ( x )   ⅆ x = x i i = 0 ∞ = x i i = 0 ∞ = x i i = 0 ∞ = f 0 1 ( x )   ⅆ x = f 0 1 ( x )   ⅆ x = f 0 1 ( x )   ⅆ x