N(N+ 1)XN−2, X∈(−1,1).2∞∞PPN(N+ 1)XN−2, S(0) = 6, XS(X) =HD

5)

n(n

+ 1)x

ⁿ⁻²

, x

∈

(−1,

1).

2

∞

P

n(n

+ 1)x

ⁿ⁻²

, S(0) = 6, xS(x) =

HD : đặt

S(x) =

n(n

+ 1)x

ⁿ⁻¹

.