0.7+9.7+99.7+999.7+…+999999999.7=?

0.7+9.7+99.7+999.7+…+999999999.7
=（1+10+100+1000+…+1000000000）-（0.3×10）
=1111111111-3
=1111111108
上面的十个数，每个数与整十数都相差0.3,所以每个数都加上0.3,最后，再减去10个0.3即可。

=(1-0.3)+(10-0.3)+(100-0.3)+……+(1000000000-0.3)
=1+10+100+……+1000000000-(0.3+0.3+0.3+……+0.3)
=1111111111-0.3×10
=1111111108

0.7+9.7+99.7+999.7+…+999999999.7
=0+0.7+10+0.7+100+0.7+...+1000000000+0.7
=1111111110+10*0.7
=1111111110+7
=1111111117

=(1-0.3)+(10-0.3)+(100-0.3)+(1000-0.3)+...+(1000000000-0.3)
=1111111111-10*0.3
=1111111108

0.7+9.7+99.7+999.7+…+999999999.7
=0.7*10+9*(1+11+111+……+111111111)
=7+9*123456789
=1111111108