I like to have my Algebra 1 students do both arithmetic and geometric sequences on the same day, and I wrote this worksheet to help them with this concept.
In the case of arithmetic sequences with unknown terms between two given terms, I have them express the first "unknown" term as u1 + d, the second unknown term as u1 + 2d, and so on. They can then set up a linear equation and solve it for d.
Likewise, with geometric sequences, I have them express the first unknown term as u1*r, the second as u1*r2, and so on. They enjoy working with the nth root button on their calculator.
This is intended for regular Algebra 1 students.
Students write directly on this worksheet.
Please download the pdf preview file first, so you can see exactly what's included; the product file is a word document, which you may edit for your students.