1-1/2-1/2*1/3-1/3*1/4-1/4*1/5-.......1/10*1/11=1-1/2-1/2+1/3-1/3+1/4-1/4+1/5...-1/10+1/11=1/11
1/2 *2/3**3/4....*10/11=1/11