Item: tExplicitArithmetic3a

Stem:

The first four terms of an arithmetic sequence are given below. Choose the algebraic expression that represents the nth term in this sequence.

7, -3, -13, -23, . . .

Options:

1. 7n - 10

2. -10n + 17

3. -10n + 7

4. 10n

5. 10n + 17

Key:

• Observable: 2. Score: 1; Evidence Value: 1.
  • Student:The expression you selected, -10n + 17, is correct. Good job!
  • Teacher:The student answered this item correctly. This item involves recognizing the explicit formula that correctly describes the "nth term" of a sequence. The fact that this sequence is decreasing contributes to this item's difficulty.
• Observable: 1. Score: 0; Evidence Value: 0.
  • Student:The expression you selected, 7n - 10, is incorrect. The sequence in the question is: 7, -3, -13, -23,. . . The expression you selected describes the sequence: -3, 4, 11, 18,. . .
  • Teacher:The student answered this item incorrectly. This item involves recognizing the explicit formula that correctly describes the "nth term" of a sequence. The fact that this sequence is decreasing contributes to this item's difficulty.
• Observable: 3. Score: 0; Evidence Value: 0.
  • Student:The expression you selected, -10n + 7, is incorrect. The sequence in the question is: 7, -3, -13, -23,. . . The expression you selected describes the sequence: -3, -13, -23, -33,. . .
  • Teacher:The student answered this item incorrectly. This item involves recognizing the explicit formula that correctly describes the "nth term" of a sequence. The fact that this sequence is decreasing contributes to this item's difficulty.
• Observable: 4. Score: 0; Evidence Value: 0.
  • Student:The expression you selected, 10n, is incorrect. The sequence in the question is: 7, -3, -13, -23,. . . The expression you selected describes the sequence: 10, 20, 30, 40,. . .
  • Teacher:The student answered this item incorrectly. This item involves recognizing the explicit formula that correctly describes the "nth term" of a sequence. The fact that this sequence is decreasing contributes to this item's difficulty.
• Observable: 5. Score: 0; Evidence Value: 0.
  • Student:You have the right idea, but the expression you selected, 10n + 17, is incorrect. Hint: it looks like you may have dropped a sign. The sequence you in the question is: 7, -3, -13, -23,. . . The expression you selected describes the sequence: 27, 37, 47, 57,. . .
  • Teacher:The student answered this item incorrectly. This item involves recognizing the explicit formula that correctly describes the "nth term" of a sequence. The fact that this sequence is decreasing contributes to this item's difficulty.

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