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
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