要懂得会还原,99变成10099x(100—1)x99 =99x99x99=(100-1)(100-1)(100-1)=100x100x100-3x100x100+3x100-1=1000000-30000+300-1= 970299
99x(100—1)x99 =99x99x99=(100-1)(100-1)(100-1)=100x100x100-3x100x100+3x100-1=1000000-30000+300-1= 970299