Let the coefficients of the middle terms in the expansion of\(\begin{array}{l} \left(\dfrac{1}{\sqrt 6}+ \beta x\right)^4, \left(1 β 3\beta x\right)^2 \text{and}\left(1 β \dfrac{\beta}{2}x\right )^6, \beta > 0,\end{array}\)respectively form the first three terms of an A.P. If d is the common difference of this A.P., then \(\begin{array}{l} 50-\frac{2d}{\beta^2} \end{array}\)is equal to _______.