2*2/1*3+4*4/3*5+6*6/5*7+8*8/7*9+10*10/9*11+12*12/11*13=1+1/3+1+1/3*5+1+1/5*7+1+1/7*9+1+1/9*11+1+1/11*13=6+(1/2)*(1-1/3+1/3-1/5+1/5-1/7+1/7-1/9+1/9-1/11+1/11-1/13)=6+(1/2)*(1-1/13)=6+6/13=6又6/13