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How to do wide-angle mathematics in the second volume of the fifth grade is more detailed, explaining the formula.
If you can't understand, just remember the rules.

If the number of items is 2 or 3, only 1 time is needed;

The number of items is 4-9, which needs 2 times;

The number of items is 10-27, which needs 3 times;

The number of items is 28-8 1, which needs 4 times;

The number of items is 82-243, which needs 5 times.