原式=2/(1x3)+2/(3x5)+2/(5x7)+2/(7x9)+2/(9x11)+2/(11x12)
=(3-1)/(1x3)+(5-3)/(3x5)+(7-5)/(5x7)+(9-7)/(7x9)+(11-9)/(9x11)+2/(11x12)
=(1-1/3)+(1/3-1/5)+(1/5-1/7)+(1/7-1/9)+(1/9-1/11)+2/(11x12)
=1+(1/3-1/3)+(1/5-1/5)+(1/7-1/7)+(1/9-1/9)-1/11+2/(11x12)
=10/11+2/(11x12)
=(120+2)/(11x12)
=61/66.