(1-1/2²)(1-1/3²)(1-1/4)²...(1-1/10²)
=(1-1/2)(1+1/2)(1-1/3)(1+1/3)(1-1/4)(1+1/4)...)(1-1/10)(1+1/10)
=(1/2)(3/2)(2/3)(4/3)(3/4)(5/4)...(9/10)(11/10)
=(1/2)(11/10)
=11/20