=(1-1/2)(1+1/2)(1-1/3)(1+1/3)(1-1/4)(1+1/4)(1-1/5)(1+1/5)……(1-1/10)(1+1/10)
=1/2·3/2·2/3·4/3·3/4·5/4·4/5·6/5·……·9/10·11/10
=1/2·11/10
=11/20
=(1-1/2)(1+1/2)(1-1/3)(1+1/3)(1-1/4)(1+1/4)(1-1/5)(1+1/5)……(1-1/10)(1+1/10)
=1/2·3/2·2/3·4/3·3/4·5/4·4/5·6/5·……·9/10·11/10
=1/2·11/10
=11/20