(3/7-1/34)*7-27/34=(3/7)*7-(1/34)*7-27/34=3-7/34-37/34=3-(7/34+37/34)=3-1=21/1*2+1/2*3+1/3*4+…+1/9*10=(1/1-1/2)+(1/2-1/3)+(1/3-1/4)+...(1/9-1/10)=1-1/2+1/2-1/3+1/3-1/4+.+1/9-1/10=1-1/10=9/10