用裂项法.因为1/5,1/9,1/13...的通项都是1/4n+1,所以1/(4n
+1)*(4n+5)=1/4*[(1/4n+1)-(1/4n+5)],原式=1/4(1/5-1/9+1/9-1/13...-1/101+1/101-1/105)=1/4*(1/5-1/105)=1/21