Arithmetic Sequence: Find Number of Terms, Second to Last Term, and Partial Sum (Reverse/Add)

From a channel Lumen includes in full: Mathispower4u (James Sousa, a maths instructor) has thousands of short worked examples covering arithmetic through calculus, statistics and linear algebra, organised to match course topics.

A 5-minute video from Mathispower4u's catalogue; the title names its topic.

Video by Mathispower4u, played through YouTube's embedded player.