1/(1*3)=(1-1/3)*1/21/(2*4)=(1/2-1/4)*1/2.1/(48*50)=(1/48-1/50)*1/2原式=1/2*[(1-1/3)+(1/2-1/4)+(1/3-1/5)+...+(1/47-1/49)+(1/48-1/50)]=1/2*(1+1/2-1/3+1/3-1/4+1/4.-1/47+1/47-1/48+1/48-1/49-1/50）=1/2*(1+...