ACT Preparation / Unit 15 ACT 1432 short lesson, about 20 seconds
Arithmetic and Geometric Sequences
Captions available
The idea
An arithmetic sequence adds a common difference d: aₙ = a₁ + (n − 1)d. A geometric sequence multiplies by a common ratio r: aₙ = a₁ · rⁿ⁻¹.
In practice
5, 8, 11 has d = 3; 3, 6, 12 has r = 2.
Where people slip
Both formulas use n − 1, because the first term has taken no steps yet.
Your place on the trail
Every lesson rests on earlier ones and opens later ones. Tap any to jump.
Builds on
Unlocks
Something wrong in this lesson? Flag it · Sources for ACT Preparation