(1-四分之一)*(1-九分之一)*.*(1-一百分之一)
=(1-1/2)(1+1/2)(1-1/3)(1+1/3)x···x(1-1/10)(1+1/10)
=1/2x3/2x2/3x4/3x···x9/10x11/10
=1/2x11/10
=11/20