Method 1: un =1/(na+b) ≥1/(a+b )×1/n, the series ∑ 1/n diverges, and the original series diverges.

Method 2: UN = 1/(na+b), UN/(1/n) = n/(na+b) →1/a (n →∞), and the series ∑ 1/n diverges, so the original.