If ${\log }_{245}175=a, {\log }_{1715}875=b,$ then the value of $\frac{1-ab}{a-b}$ is ____.

$a={\log }_{245}175=\frac{\log 175}{\log 245}=\frac{\log (5^2\times 7)}{\log (5\times 7^2)}=\frac{2\log 5+\log 7}{\log 5+2\log 7}=\frac{2\alpha +\beta }{\alpha +2\beta }$ 
$a={\log }_{1715}875=\frac{\log 875}{\log 1715}=\frac{\log (5^3\times 7)}{\log (5\times 7^3)}=\frac{3\log 5+\log 7}{\log 5+3\log 7}=\frac{3\alpha +\beta }{\alpha +3\beta }$ 
Where $\alpha =\log 5$ and $\beta =\log 7$  
Substituting a and b 
Now, $\frac{1-ab}{a-b}=\frac{5({\beta }^2-{\alpha }^2)}{{\beta }^2-{\alpha }^2}=5$