Răspuns:
1+6+6·7¹+6·7²+...+6·7²⁰²¹ = 7¹+6·7¹+6·7²+...+6·7²⁰²¹ = 7¹(1+6)+6·7²+...+6·7²⁰²¹ = 7¹·7+6·7²+...+6·7²⁰²¹ = 7²+6·7²+...+6·7²⁰²¹ = 7²(1+6)+...+6·7²⁰²¹ =....= 7²⁰²¹·(1+6) = 7²⁰²¹·7 = 7²⁰²²
√(1+6+6·7¹+6·7²+...+6·7²⁰²¹) = 49ˣ
√(7²⁰²²) = (7²)ˣ
7¹⁰¹¹ = 7²ˣ
1011 = 2x
x = 1011/2