0.1+0.2+0.3+····+0.9+0.10+0.11+0.12+0.13+····+0.98+0.99=?计算 快···

0.1+0.2+0.3+····+0.9+0.10+0.11+0.12+0.13+····+0.98+0.99
=(0.1+0.9)*9/2+(0.10+0.99)*90/2
=4.5+49.05
=53.55为什么+(0.10+0.99)*90/2要乘以90而不是99从0.10到0.99一共有90项