Unit 3, Lesson 13# Identify and use arithmetic patterns to multiply

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This lesson focuses on the study of patterns as they relate to the fact 9 = 10 - 1. Students discover that the tens digit in the product of a nines fact (? 90) is 1 less than the multiplier and that the ones digit in the product is 10 minus the multiplier. For example, in 9 × 3 = 27, 2 = 3 - 1, and 7 = 10 - 3. They also see that the digits of a product of nines fact (? 90) produce a sum of 9, as in 27 (2 + 7 = 9).