原式=[(3+1)/3]*[(4+2)/4]*[(5+3)/5]*...*[(11+9)/11]=[(2*2)/3]*[(2*3)/4]*[(2*4)/5]*...*[(2*10)/11]=[(2^9)*(2*3*4*...*10)]/(3*4*5*...*11)=1024/(10*11)=512/11