Square a 2 Digit Number, for this example 37: Look for the nearest 10 boundary In this case up 3 from 37 to 40. Since you went UP 3 to 40 go DOWN 3 from 37 to 34. Now mentally multiply 34x40 The way I do it is 34x10=340; Double it mentally to 680 Double it again mentally to 1360 This 1360 is the FIRST interim answer. 37 is "3" away from the 10 boundary 40. Square this "3" distance from 10 boundary. 3x3=9 which is the SECOND interim answer. Add the two interim answers to get the final answer. Answer: 1360 + 9 = 1369

With practice this can easily be done in your head.

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TopNewestI think it used the fact that \((x-a)(x+a)=x^2-a^2\) – Poetri Sonya · 2 years, 2 months ago

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