I keep getting “Arithmetic progression” questions wrong in Sequences & Series. What's the trap?
using the wrong count (n vs n−1) for the nth term.
The usual trap: using the wrong count (n vs n−1) for the nth term. How to avoid it: the first term uses n=1, so the (n−1) accounts for the steps taken. Go back to the definition — a sequence with a constant common difference d; the nth term is a+(n−1)d.
Key point
Arithmetic progression
Common mistake
using the wrong count (n vs n−1) for the nth term.
Memory tip
the first term uses n=1, so the (n−1) accounts for the steps taken.
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