(1-1/4)(1-1/9)...(1-1/4000000)=(1-1/2²)(1-1/3²)...(1-1/2000²)=(1-1/2)(1+1/2)(1-1/3)(1+1/3)(1-1/2000)(1+1/2000)=(1/2)(3/2)(2/3)(4/3)(3/4)...(1999/2000)(2001/2000)=(1/2)(2001/2000)=2001/4000
