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Which one of these statements about the AP 8, 5, 2, -1, … is TRUE?

Class 10Arithmetic ProgressionsAlgebra — Sequences
  • AIts 10th term is -19.
  • BIts common difference is 3.
  • CThe sum of its first 10 terms is -50.
  • DIt is not an AP, because some of its terms are negative.

Step-by-Step Solution

, not 3. , so that statement is true. , not -50. Negative terms are perfectly normal in an AP.
Answer: A) Its 10th term is -19.

Key idea: Recognising an AP

A sequence is an AP iff the difference of consecutive terms is constant.

**Test**: Compute a₂ − a₁ and a₃ − a₂. If they're equal, it's likely an AP — verify with a₄ − a₃.

**Examples**:
- 4, 7, 10, 13: differences 3, 3, 3 → AP, d = 3.
- 1, 4, 9, 16: differences 3, 5, 7 → NOT an AP (these are squares).
- 5, 5, 5, 5: differences 0, 0, 0 → AP with d = 0 (a constant sequence is an AP).
- 2, 6, 12, 20: differences 4, 6, 8 → NOT an AP.

The common difference d can be positive (increasing AP), negative (decreasing AP), or zero (constant AP).

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