1/4*1/6+1/6*1/8+1/8*1/10+1/10*/12
=1/2[1/4-1/6]+1/2[1/6-1/8]+1/2[1/8-1/10]+1/2[1/10-1/12]
=1/2[1/4-1/12]
=1/2*1/6
=1/12