裂项法:1/1*4+1/4*7+1/7*10+1/10*13+1/13*16=1/3*(1-1/4+1/4-1/7+1/7-1/10+1/10-1/13+1/13-1/16)=1/3*(1-1/16)=1/3*15/16=5/16