等差数列{an}中,公差d=1/2,前100项和是S100=45,则a1+a2+a3...+a99=?要快奥答案是19.8另外问一下为什麽我无法留言？？？

Sn=(a1+an)*n/2=45
a1+an=0.9
2*a1+99d=0.9
a1=-24.3
a100=25.2
S99=S100-a100=19.8

公差d=1/2,前100项和是S100=45
S100=a1+100(100-1)*1/2=45
=>a1=45-4950=-4905
a100=a1+99*1/2=-4905+49.5=-4855.5
a1+a2+a3...+a99
=S100-a100
=45-(-4855.5)
=4900.5