(1-2²分之1)(1-3²分之1)(1-4²分之1)...(1-10²分之1) =(1-1/2)(1+1/2)(1-1/3)(1+1/3).(1-1/10)(1+1/10)=1/2*3/2*2/3*4/3*.8/9*10/9*9/10*11/10=1/2*11/10=11/20