(5^7) (5^x+5^2)=5^16+5^x=(5^x)(5^7)+5^9 then 5^x(1-5^7)=5^9-5^16 then 5^x=(5^16-5^9)/(5^7-5^0)=(5^9-5^2)/(1-1/(5^7))=? 5^a=(5^b-5^c)/(1-1/(5^ (b-c)))=? then a=?b=?9 1 < c < b then each integer b and c then 5 ^ b=?(5 ^ b-5 ^ c)/(1-1 /(5^(b-c))) c = 2 and b = 3 ok c = 2 and b = 4 ok c =3 and b = 4 ok c = 2 and b = 5 ok c = 3 and b = 5 ok c = 4 and b = 5 ok ... c =5 and b = 6 ok ... c = 23 and b = 29 ok ... think deeply and try proof please: 1 < c < b then each integer b and c then 5 ^ b=?(5 ^ b-5 ^ c)/(1-1 /(5^(b-c))) i can see this, but I can't prove it yet! please try, how is it?