0.9+0.99+0.999+0.9999 的简便算法要简便的递等式顺便帮回答这题：0.9+49.9+199.9+1999.9+0.4

0.9+0.99+0.999+0.9999=
0.9*(1+1.1+1.11+1.111)=
0.9*4.321=
1-0.1)*4.321=
4.321-0.4321=
3.8889

1-0.1+1-0.01+1-0.001+1-0.0001=3.8889

0.9+0.99+0.999+0.9999
=0.9*(1+1.1+1.11+1.111)
=0.9*4.321
=(1-0.1)*4.321
=4.321-0.4321
=3.8889
或者0.9+0.99+0.999+0.9999
=1-0.1+1-0.01+1-0.001+1-0.0001
=4-0.1111
=3.8889
0.9+49.9+199.9+1999.9+0.4
=0.9+0.1+49.9+0.1+199.9+0.1+1999.9+0.1
=1+50+200+2000
=2251