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Math Problems and Answers of Grade Three: arithmetic progression.
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A string of numbers are arranged according to the following rules: 5, 7, 9, 1 1, …, 47, 49. Excuse me:

(1) What is the number 16 in this column?

(2) Where is this column 43?

Analysis: By observing this series, it is found that the first term is arithmetic progression with a tolerance of 2, which can be solved according to arithmetic progression's general term formula.

Solution: Solution: (1) 5+(16-1) × 2.

=5+30

=35

A: The number 16 in this column is 35.

(2)(43-5)÷2+ 1

=38÷2+ 1

=20

Answer: 43 is the 20th in this column.

Comments: arithmetic progression's general formula an=a 1+(n- 1)d is mainly used to solve the problem flexibly.