方法1
5*9+4*90+3*900+2*9000+90000
=5*(10-1)+4*(100-10)+3*(1000-100)+2*(10000-1000)+90000
=(5*10-4*10)-5+(4*100-3*100)+(3*1000-2*1000)+2*10000+90000
=10-5+100+1000+110000
=111105
方法2
5*9+4*90+3*900+2*9000+90000
=5*9+4*9*10+3*9*100+2*9*1000+9*10000
=9*(5+40+300+2000+10000)
=9*12345
=111105