The general term of is .

Here, , and n= 9 so that is

Now, for what value of i is ?

Those coefficents are, of course, , which must be equal to n+r, and which must be equal to n+ 2r, for some r, in order to form an arithmetic series. that gives you two equations to solve for n and r.Find the value of for which the coefficients of and in the expansion of form three consecutive terms of an arithmetic series.

Thanks all.