1/(1×3)=(1/2)*(1/1-1/3) 1/(3×5)=(1/2)*(1/3-1/5) 1/(5×7)=(1/2)*(1/5-1/7) ......所以原式=(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+...+1/19-1/21) =(1/2)*(1-1/21) =10/21
