=1+1/2 -(1/2+1/3)+(1/3+1/4)-(1/4+1/5)+(1/5+1/6)-(1/6+1/7)=1 +1/2-1/2 -1/3+1/3 +1/4-1/4 -1/5+1/5 +1/6-1/6 -1/7=1-1/7=6/7
