原式=1/2(1/1*2-1/2*3)+1/2(1/2*3-1/3*4)+……+1/2(1/10*11-1/11*12)
=1/2(1/1*2-1/2*3+……+1/10*11-1/11*12)
=1/2(1/1*2-1/11*12)
=65/264