=99*(99+1)=99*100=9900
99x(99+1)+100=1090
=99x99+99+100=(99+1)x99+100=100x99+100=100x(99+1)=100x100=10000
(100-1)×(100-1)+99=(100×100)-(100×1)-(100×1)+(1×1)=10000-100-100+1=9801