Solving Real Problems
Sometimes the common difference can be negative, and that's totally normal! In the sequence 9, 1, -7..., we get d = 1-9 = -8. This gives us the formula An = 17 - 8n - notice how the sequence decreases by 8 each time.
Finding specific terms is like following a recipe. For the 15th term of 18, 22, 26..., you identify A₁ = 18 and d = 4, then calculate: A₁₅ = 18 + 14(4) = 74. The key is remembering it's n−1d, not nd!
Working backwards is just as cool. If you know a term equals 122 in the sequence 8, 14, 20, 26..., you can find its position by setting up: 122 = 8 + n−16. Solving this equation gives you n = 20, meaning 122 is the 20th term.
Pro Move: When working backwards, isolate the variable step by step - don't rush through the algebra!