There is no such formula. Let us take an example to find out term independent of the variable in the binomial expansion.

 Example: Find the term independent of the variable x in the expansion (2x² + 1/x)¹²

Let the term independent of x is T_r+1

Now T_r+1 = (r+1)th term

                = ¹²C_r * (2x² )^12-r *(1/x)^r

               = ¹²C_r * 2^12-r *(x² )^12-r * 1/x^r

               =  ¹²C_r * 2^12-r *x^24-2r * 1/x^r

               =  ¹²C_r * 2^12-r *x^24-2r-r 

=> T_r+1   =  ¹²C_r * 2^12-r *x^24-3r ................1

For the term independent of x,

      24 - 3r = 0

=> 3r = 24

=> r = 24/3

=> r = 8  

So T₈₊₁ = (8+1)th term = 9th term is is independent of the variable x.

Now from equation 1, we get 

T₈₊₁  = ¹²C₈ * 2^12-8 *x⁰     

         = ¹²C₈ * 2⁴ 

=> T₉  = 16* ¹²C₈      

So in this way, we calculate the term independent of the variable in the binomial expansion.