Finding the radius of convergence is usually quick. Most lost points come from the endpoints, which need a different test each.
Find the interval of convergence of the series Σ (x − 2)n ⁄ (n · 3n), for n = 1, 2, 3, …
Step 1: ratio test. The ratio of consecutive terms tends to |x − 2| ⁄ 3 as n → ∞. The series converges when |x − 2| ⁄ 3 < 1, that is |x − 2| < 3, so −1 < x < 5 and the radius is 3.
Step 2: check x = 5. The series becomes Σ 1 ⁄ n, the harmonic series, which diverges.
Step 3: check x = −1. The series becomes Σ (−1)n ⁄ n. Its terms alternate in sign, decrease, and tend to 0, so it converges by the alternating series test.
Answer: the interval of convergence is −1 ≤ x < 5.
The ratio test says nothing at the endpoints, where the ratio equals exactly 1, which is why each endpoint needs its own test. Stopping at −1 < x < 5 is the most common way to lose this point.