1-1/2+1/3-1/4.+1/49-1/50=1-1/2+1/3-1/4.+1/49-1/50+(1/2+1/4+1/6+.+1/50)-(1/2+1/4+1/6+.+1/50)=1+1/2+1/3+1/4.+1/49+1/50-2*(1/2+1/4+1/6+.+1/50)=1+1/2+1/3+1/4.+1/49+1/50-(1+1/2+1/3+.+1/25)=1/26+1/27+.+1/49...