The middle term in the expansion of ${\left(3x-\dfrac{{x}^{3}}{6}\right)}^{9}$ is

The number of irrational terms in the expansion of$( \sqrt [ 8 ] { 5 } + \sqrt [ 6 ] { 2 } ) ^ { 100 } ,$ is

In the expansion of $(a-b)^{n},n\ge 5$, if the sum of the $5^{th}$ and $6^{th}$ terms is zero, then $a/b$=

The middle term in the expansion of $(a x)^n$ is

The value of the sum $\sum _{ j=0 }^{ 8 }{8 \choose j}{ \frac { 1 }{ (j+1)(j+2) } }$ is