1-1/2-(1/2)(1/3)-(1/3)(1/4)-……-(1/10)(1/11)
=1-1/2-[1/2-1/3]-[1/3-1/4]-……-(1/10-1/11)
=1-1/2-1/2+1/3-1/3+1/4-……-1/10+1/11
=1/11
1-1/2-(1/2)(1/3)-(1/3)(1/4)-……-(1/10)(1/11)
=1-1/2-[1/2-1/3]-[1/3-1/4]-……-(1/10-1/11)
=1-1/2-1/2+1/3-1/3+1/4-……-1/10+1/11
=1/11