Sum of Arithmetic Sequence Given 4th and 12th Term

This video explains how to determine a partial sum of an arithmetic series gives two terms. The terms are not consecutive and the first term is not given.

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Formula for a Geometric Sequence Given 6th and 9th Terms (System-Fraction)

This video explains how to determine the general formula for a geometric sequence given two terms. The terms on are consecutive and the 1st term is not given.