Discussion

The missing numbers in the pattern 12, 14, 17, 13, 8, 14, 21, _, _ are respectively :

Answer: B

The missing numbers are 13, 4.

Look at the differences between successive terms:

12 → 14 → 17 → 13 → 8 → 14 → 21: +2, +3, −4, −5, +6, +7.

The size of the step rises by one each time, and the sign follows the pattern two additions, two subtractions. So the next two steps are −8 and −9:

\[ 21 - 8 = 13, \qquad 13 - 9 = 4 \]

Adding instead of subtracting, or using a constant step, gives the other pairs.

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