venneblock,

Preaching to the choir. I completely agree. I was just posing a possible reason for leaving it off.

]]>In any event, I find this trick handy when calculating the squares of numbers ending in 5. For example, 35 squared is 35 * 35. Tens digit is the same for any squared number and 5 + 5 always sums to 10. So 3 * 4 = 12, and 5 * 5 = 25, so 35 squared = 1225.

I also use that for problems like 33 * 34 (or something similar). Since I know 35 * 35 = 1225, I can subtract 35 to get 1190 which is 35*34. Subtracting 34 yields 1156 which is 34 * 34. Finally subtracting 34 again makes 1122, the product of 33 * 34. My students are amazed I can do that “hard” problem in my head.

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