用裂项法：原式= 1/(1×6)+1/(6×11)+1/(11×16)+……+1/(101×106)= (1/5)×(1-1/6)+(1/5)×(1/6-1/11)+(1/5)×(1/11-1/16)+……+(1/5)×(1/101-1/106)= (1/5)×[(1-1/6)+(1/6-1/11)+(1/11-1/16)+……+(1/101-1/106)]= (1/5)×(1-1/106)= 21/106
