1/3 1/2 5/11 7/18 1/3
=
1/3 3/6 5/11 7/18 9/27
=(2*1-1)/(1*1+2),(2*2-1)/(2*2+2),(2*3-1)/(3*3+2),(2*4-1)/(4*4+2),(2*5-1)/(5*5+2)
所以填(2*6-1)/(6*6+2)=11/38