AI response
Recommended AI: ChatGPT, Claude
🔍 There's a reason behind every miss — let's catch yours together.
How your misses sort out
| Item | Cause | Why |
|---|---|---|
| Q3 sequence sum | Application slip | Knew the formula, used n instead of n-1 for the last term |
| Q5 sequence sum | Application slip | Same off-by-one on the index |
| Q7 sign error | Carelessness | Dropped a minus while expanding |
| Q9 recurrence | Concept gap | Couldn't set up the relation at all |
Top weakness · index off-by-one (application slip)
Two of four misses are the same "where does the term start/end" mistake, and it strikes on the highest-value sum problems. It's not that you don't know the formula — you apply it on autopilot, so it's a cheap fix with a big payoff.
The trap you'll hit again
Sum and recurrence problems collapse for you the moment the index shifts (starts at k=0, or asks for "the last term"). Whenever you see a sum sign, you'll be tempted to plug n straight in.
Your 3 fixes
- Index ritual (5 min/problem): before solving any sum, write out the first and last terms by hand. Forces the boundary into view.
- Variant drill (15 min): redo Q3 and Q5 with the start index changed to k=0 and k=2. Same shape, moved boundary.
- Recurrence setup (20 min): relearn "how to translate a word relation into a_n and a_(n-1)" — that's the one true concept gap.
Self-test
- For a_n = 2n-1, what is the sum of the last 3 terms up to n=10?
a_8+a_9+a_10 = 15+17+19 = 51 (watch the boundary).
- Write the recurrence for "each term is the previous term plus 3, starting at 4".
a_1=4, a_(n)=a_(n-1)+3.





