9999.9+999.9+99.9+9.9=10000-0.1+1000-0.1+100-0.1+10-0.1=11109.6
9999.9+999.9+99.9+9.9=(10000-0.1)+(1000-0.1)+(100-0.1)+(10-0.1)=(10000+1000+100+10)-0.1×4=11110-0.4=11109.6
运用交换律计算:10000-0.1+1000-0.1+100-0.1+10-0.1=11110-0.4=11109.6