0.9+9.9+99.9+999.9+9999.9简便计算为什么每次加0.1最后又减0.5求解读

0.9+9.9+99.9+999.9+9999.9
=0.9+0.1-0.1+9.9+0.1-0.1+99.9+0.1-0.1+999.9+0.1-0.1+9999.9+0.1-0.1
=（0.9+0.1）-0.1+（9.9+0.1）-0.1+（99.9+0.1）-0.1+（999.9+0.1)-0.1+(9999.9+0.1)-0.1
=1-0.1+10-0.1+100-0.1+1000-0.1+10000-0.1
=11111-0.5
=11110.5
减的0.5是因为式子中加了5个0.1