Radius and interval of convergence of a power series, using ratio test, ex#1

From a channel Lumen includes in full: Steve Chow (blackpenredpen) is a long-time calculus teacher who works integrals, derivatives, series, special functions and precalculus problems in full on the whiteboard, including multi-hour practice marathons.

A 13-minute video from blackpenredpen's catalogue; the title names its topic.

Video by blackpenredpen, played through YouTube's embedded player.