If the 4th term in the expansion of (ax+1/x)n is 52, then

It is given that the fourth term in the expansion ofax+1xn is 52; therefore  nC3(ax)n−31x3=52⇒nC3an−3xn−6=52[ R.H.S. is independent of x]Putting n=6in(1),we get  6C3a3=52 or a3=18 or a=12