2*(10^n-1)=2*10^n-2
而(2+10^n-1)(10^n-1)=(10^n+1)(10^n-1)=10^2n-1 (即将两式相加)
故(10^n-1)^2=(10^2n-1)-(2*10^n-2)=(10^n-2)*10^n+1
2×9=18 9²=81 2×99=198 99²=9801 2×999=1998 999²=998001 .
2×9=18 9²=81 2×99=198 99²=9801 2×999=1998 999²=998001 .
规律显而易见,求理论证明.
规律显而易见,求理论证明.
数学人气：431 ℃时间：2020-04-03 20:10:34
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