(1+1/2)*(1+1/4)*(1+1/6)*.*(1+1/20)*(1-1/3)*(1-1/5)+(1-1/7)*.*(1-1/21)=(3/2)(5/4)(7/6)...(21/20)(2/3)(4/5)(6/7)...(20/21)=(3/2)(2/3)(5/4)(4/5)...(21/20)(20/21)=1