N , X∈(−1,1)1∞XNXHD

2) - 20041028-THAYHOA-BAI3
20041028-THAYHOA-BAI3

2)

n

, x

∈

(−1,

1)

1

∞

x

n

X

HD:

f(x) =

n

, f(0) = 0. Đạo hàm hai vế.