1*6分子1+6*11分之1+11*16分之1+.+101*106分之1
=1/5(1/1-1/6)+1/5(1/6-1/11)+1/5(1/11-1/16)+.+1/5(1/101-1/106)
=1/5(1/1-1/6+1/6-1/11+1/11-1/16+.+1/101-1/106)
=1/5(1-1/106)
=1/5*105/106
=21/106