九. ρ= lim
x = ±1 时级数发散, 故收敛域 x ∈(-1, 1).
S(x)= ∑
= 2∑
= 2S1(x) - x/(1-x) x ∈(-1, 1).
∫<0,x>S1(t)dt = ∑
= ∑
S(x) = 2[x^2/(1-x)]' - x/(1-x)
= (2x-x^2)/(1-x)^2- x/(1-x) = x/(1-x)^2 x ∈(-1, 1).
我大一,好像不会