The sum of coefficients of the  expansion 1x+2xn is 6561 . The coefficient  of term independent of x is

Tthe sum of the oefficients of the expansion1x+2xn=6561  Therefore (1+2)n=38 ⇒3n=38⇒n=8  Let (r+1) th term is independent of x .   Therefore, Tr+1=Cr   81x(2x)8-r                                =C  8  r    2   8-r  x8-2r  Since this term is independent of x , then 8-2r=0⇒r=4  Therefore, coefficient of independent term is T5=C4   8·24=16·C4   8