ffirs.indd

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228 Appendix J
Why does it work? Try using 70 as a reference number and you will
see we are dealing with something we already know.
+5
+5
70 75 × 75 = 80
5,600



  • 25
    5,625
    Try the problem for yourself using 80 as a reference number. It
    works out the same.
    So for 135^2 (135 × 135) the front part of the number is 13 (in front
    of the 5). We add 1 to get 14. Now multiply 13 × 14 = 182 (using
    the shortcut in Chapter 3).
    We square 5, or just put 25 at the end of our answer.
    1352 = 18,225
    For that we used either 130 or 140 as a reference number.
    Try squaring 965 in your head.
    Ninety-six is in front of the 5.
    96 + 1 = 97
    96 × 97 = 9,312
    Put 25 at the end for the answer.
    9652 = 931,225
    Th at is really impressive.
    bbapp10.indd 228app 10 .indd 228 1 1/5/07 11:39:00 AM/ 5 / 07 11 : 39 : 00 AM

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