Why redo wrong items?

Grouping the same mistake together shows exactly which step of the formula went wrong.

How should a wrong answer be recorded?

Note the faulty step, such as a missing +1 or a forgotten division by (1 − r), then recompute.

IB Maths AA: Sequences and Series (10 Error-Correction Items)

2026-11-08

Ten error-correction items: each shows a common wrong working first, then asks for the correct value, focusing on the n − 1 power, dividing by (1 − r) and the final +1 when counting terms.

Focus　Find which step of the formula went wrong, then recompute and check the figure.

01　Paper details (check answers afterwards)

Item	Detail
Questions	10
Suggested time	30
Difficulty mix	Error redo 10
Chapter	Chapter: IB Maths AA: Sequences and Series
Level	IB Maths AA (SL/HL)

02　Questions (10 items)

Error redoA student works out the 15th term of an arithmetic sequence (a = 7, d = 4) as 7 + 15 × 4 = 67. What is the correct value?
Error redoA geometric sequence has first term 3 and ratio 2; a student gives the 6th term as 3 × 2^6 = 192. What is the correct value?
Error redoFor a geometric series with a = 12 and r = 0.4, a student computes the infinite sum as 12 × (1 − 0.4) = 7.2. What is the correct sum?
Error redoAn arithmetic series has first term 5, last term 45 and 9 terms; a student answers 9 × 50 = 450. What is the correct sum?
Error redoHow many terms are in the sequence 4, 9, 14, …, 94? A student answers 18. What is the correct count?
Error redoAn arithmetic series has a = 2 and its first 10 terms sum to 155; a student computes 155 ÷ 10 − 2 = 13.5, divides by 9 and answers d = 1.5. What is the correct difference?
Error redoA sum of 8,000 earns 5% compound interest for 3 years; a student computes 8 000 × 1.15 = 9,200. What is the correct amount?
Error redoConvert the recurring decimal 0.272727… to a fraction; a student answers 27/100. What is the correct fraction?
Error redoA geometric series has first term 9 and sum to infinity 36; a student answers that the ratio is 0.25. What is the correct ratio?
Error redoAn arithmetic sequence has a = 5 and d = −3; a student answers that the 8th term is 5 − 3 × 8 = −19. What is the correct value?

03　Answers and one-line notes

#	Answer	One-line note
1	63	The general term is a + (n − 1)d, so the difference is added 14 times: 7 + 14 × 4 = 63.
2	96	The nth term is a r^(n − 1), so the 6th term uses 2^5 and equals 3 × 32 = 96.
3	20	The sum to infinity is a ÷ (1 − r) = 12 ÷ 0.6 = 20; multiplying by (1 − r) is not the same as dividing.
4	225	The sum is n ÷ 2 × (a + l) = 4.5 × 50 = 225; leaving out the division by 2 doubles the answer.
5	19	The number of terms is (94 − 4) ÷ 5 + 1 = 19, because both end values count and the +1 is required.
6	3	155 = 10 ÷ 2 × (4 + 9d) gives 31 = 4 + 9d, so d = 3; 15.5 is the mean of the terms, not a gap from the first term.
7	9,261	Compound interest multiplies by 1.05 three times: 8 000 × 1.05^3 = 9,261, while 1.15 would be simple interest.
8	3/11	The repeating block has two digits so the denominator is 99, and 27/99 reduces to 3/11; over 100 gives a terminating decimal.
9	0.75	36 = 9 ÷ (1 − r) gives 1 − r = 0.25, so r = 0.75; the value 0.25 is 1 − r, not r.
10	−16	The 8th term subtracts the difference 7 times: 5 − 21 = −16, and the multiplier is always n − 1.

04　What to do after marking

After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
Repeat the same item two days later; move to the next paper only when both attempts are correct.
Parents only need the answer and note columns; there is no need to explain every item.

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Questions and answers are written originally by the TutorZone team and are not taken from any school, publisher or examination authority paper. Every value is checked by computer.